Dissection Manual for Dental Students Sujatha Kiran
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1DISSECTION MANUAL for Dental Students
2DISSECTION MANUAL for Dental Students
Sujatha Kiran PhD Professor and Head Department of Anatomy and Academic Vice Principal MNR Medical College Sangareddy, Andhra Pradesh, India
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Dissection Manual for Dental Students
First Edition: 2012
9789350255964
Printed at
4Dedicated to
My Father Sri Pendyala Nageswara Rao
5Preface
Anatomy is the study of structure of the human body. The gross anatomy describes the structures as they are seen in the body. To study them, dissections are performed on the cadaver.
Human anatomy conventionally is studied as regional anatomy. The body is divided into six to seven regions and the structures located in a particular region are described in dissection manuals and standard textbooks.
The dissection manual is a practical guide in the dissection hall to unfold the position, relations and functions of the structures seen. This book is aimed at students pursuing dental course. They need to do dissections of Head and Neck and Brain. All other important organs are expected to be studied from the dissected body.
Knowledge of the basic architecture of the body is an essentiality to understand the structure of any region. So, an introductory chapter is given here along with notes on how to perform dissection.
Chapter on Head and Neck starts with an introductory note. This gives an outline of arrangement of structures in that part and their functional specialization. This is followed by smaller segments of dissection regions. Each region describes the structures seen in that particular region with a pictorial depiction. This carries information needed to identify the structures.
Chapter on Central Nervous System (CNS) gives a detailed dissection and description of the concerned parts. Knowledge of the CNS is very much essential for any doctor treating the patients. Dental students can collaborate with the medical students in removing the spinal cord.
Living and surface anatomy is incorporated at places to make the student understand the correlation between the structures that are seen in the cadaver and the functions they perform in the living.
This book is aimed at undergraduate student. After seeing and identifying the structures, they are expected to read the details from a standard textbook.
Sujatha Kiran
11Introduction  
 
ANATOMICAL TERMINOLOGY
The human body is obtained after death after clearing all the legal formalities. Now the cadaver is properly embalmed with appropriate embalming fluid and well preserved. All the human beings have the same structure, but can still differ. These differences are called variations. Each and every structure in the body is named. These names are coined, generally with some meaning and are universally accepted by the scientists of this field. This chapter explains the structures that are encountered in the dissection and the common terminology used.
ANATOMICAL POSITION
Anatomical position () is described as one where the person stands upright with the upper limbs hanging by the sides and the palms facing forwards. All the structures within the body are described as they lie in the anatomical position, though in reality the body lies horizontal on the table.
ANATOMICAL TERMS
Anatomical terms are the names used to identify the structures in the body. Try to study and practice the following terms.
Median: Denotes the midline of the body. This is a definitive term. All the other terms are relative.
Superficial: Nearer to the skin, e.g. the veins are superficial to the deep fascia.
Deep: Away from the skin, e.g. the veins are deep to the skin.
Superior/cephalic: Nearer to the head, e.g. eyes are superior to the nose.
Inferior/caudal: Nearer to the tail, e.g. the diaphragm is caudal to the heart.
Anterior/ventral: Nearer to the front, e.g. the heart is ventral to the vertebrae.
Dorsal/posterior: Towards the back, e.g. the vertebrae lie posterior to the heart.
Medial: Nearer to the midline, e.g. the trachea lies medial to the lungs.
FIGURE 1: Anatomical position
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Lateral: Away from the midline, e.g. the lungs lie lateral to the trachea.
External: Nearer to the outside, e.g. the pericardium lies external to the heart.
Internal: Nearer to the inside, e.g. the heart lies internal to the pericardium.
Proximal: Nearer to the body, e.g. the arm lies proximal to the forearm.
Distal: Away from the body, e.g. the forearm lies distal to the arm.
Radial, ulnar, tibial, fibular are the terms used to denote the structures nearer to those bones, e.g. ulnar artery—the artery related to the bone ulna.
Superolateral, inferomedial, anteroinferior, posterosuperior are the terms used to depict an accurate position of a structure in the body, e.g. the subclavian artery lies posterosuperior to the subclavian vein.
Palmar means towards the palm of the hand, e.g. the palmar aponeurosis lies in the palm of the hand.
Plantar means towards the sole of the foot, e.g. the plantar aponeurosis lies in the sole of the foot.
ANATOMICAL PLANES ()
Sagittal plane: Cutting through the body anteroposteriorly.
Midsagittal plane or median: Cutting through the body exactly in the midline.
FIGURE 2: Anatomical planes
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Parasagittal plane: Parallel to the sagittal plane.
Coronal plane: This is perpendicular to the sagittal plane - cutting the body from one side to the other.
Transverse plane: Cutting the body horizontally—this is generally used to study the organs for diagnostic purposes like MRI, ultrasound studies.
TERMS OF MOVEMENTS
Terms of movements (perform them on your body and appreciate).
Flexion is when two ventral surfaces/anatomical or embryological approximate, flexion is also described as closing up of an angle, e.g. forearm touching the arm at the elbow joint. In lower limb back of the leg touching the back of the thigh is flexion. Here two embryological ventral surfaces approximate each other.
Extension is when two dorsal surfaces approximate or opening up of an angle, e.g. bringing arm and forearm into a straight line. In lower limb bringing the thigh and leg into straight line is extension.
Abduction is going away from the midline, e.g. the upper limb moving away from the body, in relation to the movement of the fingers, the imaginary midline passes through the middle of the middle finger, move the fingers away from the middle finger. The hand moving away from the body or moving towards the radial side is called radial deviation.
Adduction bringing the part nearer to the midline, e.g. bringing the upper limb nearer to the body, move the fingers of the hand, back to touch the middle finger. When the hand moves towards the body it is called adduction or ulnar deviation.
Internal/medial rotation—where the ventral surface of the part turns medially, e.g. turn the hand to the back, holding the humerus in your hand, feel the humerus turning around a vertical axis.
External/lateral rotation is where the ventral surface of the part turns laterally, e.g. turn the hand out by holding the humerus. This is a rotation around a vertical axis.
Protraction is where a part moves bodily forwards, e.g. move the shoulder forwards.
Retraction is where a part moves backwards, e.g. bracing the shoulders.
Supination is where the palm lies in normal anatomical position, when the body is put on the table facing forwards it is said that the body is in supine position.
Pronation is where the palm faces backwards or where the body is put on the table with face touching the table.
Dorsiflexion is used only in relation to foot. When the foot moves towards the front of leg it is called dorsiflexion.
Plantar flexion is when the foot is lifted off the ground with the sole facing backwards.
Circumduction is where the part moves in a circular motion creating a cone, e.g. move the upper limb totally in a circular motion.
Opposition is a special movement of the thumb, where the thumb touches the other fingers.14
 
STRUCTURES ENCOUNTERED IN DISSECTION
This chapter explains the structures you encounter as you do your dissection.
SKIN
Skin is the organ which covers the body totally. While doing the dissection you will realize number of specializations within the skin. The skin over the palms and soles is very thick and is connected to the deeper fascia by means of thick connective tissue septa and the gaps between the septa is filled with fat. So it is difficult to pull the skin of the palms and soles. It is devoid of hair follicles, sebaceous glands and arrectores pilorum muscle. Skin of the scalp is also connected to the deeper fascia by connective tissue septa. Skin of the scalp, the pubic region and the axillary region is heavily laden with hair follicles, sebaceous glands and sweat glands. The skin on the ventral aspect of the body is lighter and thinner, can easily be pulled from the underneath fascia. Pull it and see. Skin on the back is darkly colored, thick and firmly attached to the deeper fascia. While reflecting the skin you have to keep these features in mind. See the skin flexure creases near the wrist, elbow, knee etc. These are due to constant folding of the part and pull of the connective tissue beneath. Observe dermal ridges and flexor creases on the palmar and plantar skin. These are genetically determined and are formed within the intrauterine life itself. The study of these is called dermatoglyphics. The dermal ridge patterns are unique to each individual. This is used in forensic study.
Nerve supply of the skin (): The skin develops from dermatome part of the somite. As it develops it drags the nerve supply along with it. So the nerve supply of the skin denotes from which somatic segment it is derived. This arrangement is definitive throughout the body. Knowledge of this pattern is very much essential to diagnose the damage to a particular spinal nerve.
Langer's line (): Skin is the first structure to be cut in any operation. The collagen fibres beneath the skin follow a definitive pattern. If collagen fibres are cut across, they form a thick scar in wound repair. Instead, if a cut is made along the fibre direction, during wound repair there will be minimal scar formation. So the knowledge of the arrangement of collagen fibres beneath the skin is very much essential for a surgeon. This arrangement of collagen fibres beneath the skin is called Langer's lines.
FIGURE 3: Dermatomes
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FIGURE 4: Langer's line
SUPERFICIAL FASCIA ()
As you reflect the skin the next structure you encounter is the superficial fascia. It is the connective tissue deep to the skin. The arrangement of tissue is variable in different parts of the body. In most of the places it is loose and yielding. Try to pull the skin on your forearm. The area on which the skin slides is the superficial fascia. Note that on the back of your body it is not easy to pull the skin, because the connective tissue here is dense connective tissues. Note that in the palms, soles and scalp skin is very thick. That is because the connective tissue here connects the skin to the deep fascia beneath, and is divided into number of fat laden compartments. This forms a thick cushion in these regions.
FIGURE 5: Superficial fascia
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The superficial fascia is generally filled with fat. The amount of fat present varies from person to person. Even in a very thin person, the gluteal region, anterior abdominal wall and greater omentum are heavily laden with fat. The cutaneous blood vessels and nerves traverse through the superficial fascia.
Cutaneous veins: These are thin bluish to black tubes seen in the superficial fascia. At the time of death the arteries undergo a wave of contraction and push the blood into the venous system. It gets stagnated there and gives a black coloration. Make a tight fist and see the veins on the back of your hand. These appear greenish in color. They form a superficial prominent definitive system of veins in the upper limb and lower limb. These are generally made use to draw blood and inject fluids into the body. In other parts of the body they accompany the medium and small sized arteries as venae comitantes.
Cutaneous arteries: These are very fine arteries. They are always accompanied by venae comitantes. These are veins accompanying the arteries. The cutaneous arteries are small and difficult to identify unless they are injected with red lead. Nowadays this practice of injecting the arteries is discontinued. Generally the cutaneous arteries are identified by identifying their venae comitantes.
Cutaneous nerves: Cutaneous nerves generally accompany cutaneous vessels in the body. In the limbs, cutaneous nerves pierce the deep fascia at definitive places, traverse long distance, branch and supply a big area of skin. These are sensory nerves. They carry cutaneous sensations like pain, touch pressure type of sensation from skin.
Lymphatics: These are very thin vessels and difficult to trace. The lymph nodes into which the lymphatics drain are definitive in their position. These can be easily identified along with the drainage vessels. The nodes appear deep brown in color and oval to round in appearance.
Cutaneous muscles: These are muscles present within the dermis of the skin. These are smooth muscle fibres called arrectores pilorum muscle. They cannot be identified in dissection but they can be seen under microscope. The panniculus carnosus group of muscles is a subcutaneous sheet of muscle. They are inserted into the skin and can move it. So in places where they are present you have to take a great care while reflecting the skin, e.g. muscles of facial expression, dortos muscle of scrotum and palmaris brevis of palm.
DEEP FASCIA
The thick white glistening protective sheet that you see deep to the superficial fascia is the deep fascia. It is made of collagen fibres. The collagen fibres are longitudinally arranged in limbs where it covers the musculature. At many a places it is thick and gives attachment to muscles, e.g. fascia lata of thigh gives attachment to gluteus maximus and tensor fascia lata muscle. It is a thick owen structure when it tries to protect the tendons crossing the bones. Here it is called a retinaculum, e.g. flexor retinaculum, peroneal retinaculum. The deep fascia sends in septa, to separate different functional groups of muscles. This can be easily identified by pulling the deep fascia outwards while reflecting the fascia.
SKELETAL MUSCLE
Once you reflect the fascia the reddish brown muscle mass you see is skeletal muscle. It is under voluntary control. All these muscles have at least one bony attachment. Generally they extend from one bone to the other, crossing a joint or more than one joint. But the muscles over the face get attached from the bone to the skin. They move the skin of the face and are called muscles of facial 17expression. They belong to the panniculus carnosus group of muscles. This is the muscle sheet which lies beneath the skin in carnivores and help them to shrug. For the muscles of tongue the distal attachment is mucous membrane, the muscles of eyeball the distal attachment is sclera.
Arrangement (): The muscles form the major bulk in limbs; they are arranged in compartments as functional groups, e.g. the flexors of the arm occupy the anterior compartment. The compartments are separated from each other by septa. Each compartment has its own set of neurovascular bundle which supplies blood and nerves. The neurovascular bundle is supported by loose connective tissue, which you need to clean to identify the branches. The nerve enters the muscle bellies on its under surface.
FIGURE 6: Skeletal muscle arrangement section through arm
Fibre arrangement: In skeletal muscle, the muscle fibres are cylindrical in shape. They are covered by connective tissue. The muscle fibres reach the bone through this connective tissue. The force generated by the muscle fibres reach the bone through this connective tissue fibres called Sharpey's fibres. The proportion of connective tissue and muscle fibres is variable. The sartorius is muscular throughout its extent, the flexor muscles of forearm have long tendons, anterior abdominal muscles are replaced by big aponeurosis near their insertion. The arrangement of muscle fibre to the connective is variable. The muscle acquires their shape based on the connective tissue. They can be strap muscles when fibres are parallel and connective tissue is less, e.g. sartorius; spindle shaped or fusiform, when the muscle fibres from a belly in the center and reach the bone through ends, e.g. biceps, digastrics; unipennate where all the muscle fibres get attached to a tendon from one side, e.g. flexor pollices longus; bipennate where muscle fibres get attached to a central tendon from two sides, e.g. dorsal interossei; multipennate where the muscle fibres are short and get attached to number of central tendons which unite together to from a big tendon near the insertion, e.g. deltoid; is circumpennate arrangement, where all fibres converge from all sides to a central tendon, e.g. tibialis anterior.
Nomenclature: Muscles are named for convenience of understanding based on number of parameters. They are named based on shape—deltoid (triangular), quadratus (quadrangular). Rhomboideus (diamond shaped) teres (rounded) lumbrical (worm like) size—major (big), minor (small), longus (long), brevis (short), latissimus (broad); depending upon number of heads—biceps (two heads), triceps (three heads), quadriceps (four heads), digastric (two bellies); based upon position—superficialis (nearer to skin), profundus (deeper to the skin), externus (near to outside); based on location—anterior (infront), dorsal/posterior (back), lateralis (external side), superior (nearer to the head), inferior (nearer to the tail), interosseous between the bones, pectoralis (over the chest), brachialis (in the arm), femoris 18(in the thigh), oris (in the mouth), oculi (in relation to the eye); depicting the attachment—sternocleidomastoid (from sternum to mastoid process), based on action—flexor (performs flexion), extensor (performs extension), abductor (performs abduction), adductor (performs adduction), supinator (performs supination), pronator (performs pronation); and a combination of any of the above, e.g. flexor carpi radialis brevis—a small muscle which performs flexion of the carpal bones on the radial side.
Living anatomy: Muscle testing is a common practice to assess the damage of a muscle. As they are in groups it is difficult to assess a single muscle damage. But a group performance can easily be assessed. If a person is asked to perform a movement against resistance the muscle will stand out, e.g. try to approximate the forearm to the arm by pushing the forearm with the other hand. The biceps of the arm will stand out, as it is a flexor of the forearm at the elbow joint. In the dissection hall try to test as many muscles as possible.
Muscle action: Many factors come into play when a muscle contracts to bring about a particular action. To understand this one should be familiar with axis. The axis is the least mobile line or plane in a joint while performing a movement. Any muscle which crosses the axis exactly perpendicular will have the best powerful action, as they move away from the center the action will be weaker. Based upon this factor a muscle can be described as prime mover, e.g. biceps is a prime mover for flexion of forearm on arm at elbow. The muscles which help this prime mover are called agonists, e.g. brachialis and pronator teres are agonists to perform flexion. The muscles which cross the axis on the opposite side of the agonists are antagonists, e.g. the triceps which brings about extension of the forearm at elbow is an antagonist to biceps. When an action is to be performed against gravity both progravity and antigravity muscles perform the action. It is like putting a break to a car when we are riding on a slope. It is called paying out rope action, e.g. lowering a heavy weight to the ground. Here both biceps and triceps contract. When both agonists and antagonists contract to bring about an intricate powerful movement it is said that they are synergists, e.g. while making a fist both flexors and extensors of the hand contract.
Biomechanics: Kinesiology is the subject which deals with the biomechanics of the joints. Here only basics are mentioned for understanding the muscle movement. Each muscle generates a force when contracts. This force is passed on to the bone through Sharpey's fibres. A combination of all forces generated by all the muscles result in the net movement produced at a particular joint. The point of insertion is the fulcrum.
Spurt force: When a muscle is inserted into a part nearer to the joint, and when it contracts it pulls the distal segment. This moves with a greater range. This is spurt force, e.g. the biceps which is inserted into the proximal part of the radius, causes the range in flexion of the forearm.
Shunt force: When a muscle is inserted into a distal segment of the bone it pushes the proximal part backwards and creates shunt (like locos) force, e.g. the pronator teres, brachioradialis by pushing the radius towards the elbow joint they create a stabilizing force.
Spin force: When a muscle spirals or runs transversely and gets attached to a convexity, it produces a spin force. It produces a rotatory movement (like spinning a cricket ball), e.g. infraspinatus at the shoulder joint and pronator teres between radius and ulna.
Soleal pump: This is a special feature seen in soleus. Veins form a plexus within the soleus muscle of the leg. It functions as a peripheral heart and aids in the venous return.
Sesamoid bone: These are bones which develop in the muscles. When a muscle or a tendon crosses a joint too closely, it replaces the capsule and develops bone within its substance. This prevents 19friction and wearing away of the muscle. These can be bony or cartilaginous, e.g. patella or knee cap in the tendon of quadriceps femoris muscle (feel it), number of sesamoid bones in tendons of sole.
Synovial sheaths (): When tendons pass over bones and joints they are protected by synovial sheaths. These are bags of connective tissue filled with fluid. This prevents friction of the tendons over the bones, e.g. flexor sheaths of long tendons of fingers.
FIGURE 7: Formation of synovial sheath—diagrammatic representation
Bursa: Number of muscles are separated from the nearby bone by bags of synovial fluid called bursa. This gives a play for the muscles, e.g. subscapular bursa.
Nerve supply: Though conventionally it is said that motor nerve, supplies a muscle and it brings about contraction, in reality both motor and sensory nerves supply the muscle fibres. The motor nerve initiates the contraction of muscle, whereas sensory fibres carry proprioceptive/stretch sensations from the muscle. The receptor here is called muscle spindle. The knowledge of this is essential to initiate contraction.
Motor unit: Each nerve has number of fibres and each nerve fibre supplies as many as 100 muscle fibres. This is called a motor unit, so stimulation of each nerve fibre initiates a contraction of 100 muscle fibres. More and more muscle fibres will be involved in contraction depending upon the force required in contraction.
Hilton's law: The nerve which supplies a group of muscles that act on a joint will give a branch to supply the joint. These are sensory fibres and carry stretch sensations from tendons, capsule and ligaments.
The spindle shaped muscles are generally supplied in the center and into their bellies, e.g. biceps. In muscles where the fibres are spread out, the nerve has a long course and it gives branches along the length of the muscle, e.g. accessory nerve to trapezius. Generally all the muscles are supplied on their ventral aspects.
Blood supply: The medium sized arteries accompanied by venae comitantes supply the muscles along with the nerves. It is called a neurovascular bundle. The tendons are supplied by arteries which supply the joints. When the tendons are too long they have their own neurovascular bundle, e.g. vincula longa and brevia of flexor digitorum superficialis and profundus.20
CARDIAC MUSCLE
The cardiac muscle is a specialized muscle specific for heart. It runs in layers to control the contraction of different chambers.
SMOOTH MUSCLE
Smooth muscle is involuntary musculature, present in the internal organs. Generally they are arranged in layers. They produce peristaltic movement in gastrointestinal tract, and the uterine musculature expels the fetus during parturition.
BONE
The muscles are attached to bones. The next structure you would come across in the body is bone. The study of bones is called osteology. It is a big field and only the details needed for dissection is covered in this book.
The bones form the framework of our body. The dried bones taken out of a human body are articulated into a skeleton with the help of screws, wires and corks. It is essential to go to a skeleton and study the bones, to understand the location of the joints and the position of muscle attachment. You need to see the direction of muscle fibres to understand their actions.
Skeleton (): Go to a skeleton and see the arrangement of bones. The central axis of the body is made up of the vertebral column. The skull, the ribs and the pelvis form the axial skeleton, the bones of the limbs form the skeleton of the appendages, the appendicular skeleton.
Shapes of the bones: Note that the bones are of different shapes and sizes. Observe the bones on the vault of the skull, scapula and hip bone, they are flat bones. See the bones forming the base of the skull and the vertebrae, they are irregular bones. See the bones of the limbs and ribs, they are long bones, they have a shaft and two articular ends. Look at the carpals and tarsals, they are short bones. See a midline cut skull, you can see number of cavities within the bone, they are pneumatic bones.
 
Contours of the Bones
While doing dissection you can appreciate that the bones present two parts, the part which gives attachment to muscles and ligaments, and the other part which takes part in the formation of joints.
Intra-articular parts: These are the parts which contribute to the joint formation and are smooth, and show different shapes. Head is rounded structure, present towards the ends of the bones, e.g. head of femur, head of humerus. Neck is the part that follows the head, e.g. neck of the humerus, neck of the mandible. Condyles are semicircular bulges at the ends of the bones, e.g. femoral condyles, tibial condyles. Trochlea is a pulley shaped elevation, e.g. trochlea on humerus. Eminence is a central elevation, e.g. intercondylar eminence on tibia. This is the elevated area between the two tibial condyles.
Markings due to muscle pull: The part of the bone which gives attachment to muscles and ligaments raise elevations due to the pull of the muscles, and are given different names. Trochanter is a big elevations on the bone, e.g. greater trochanter and lesser trochanter of femur. Tubercles, tuberosity, protuberance, supracondyles are the words commonly used to denote small elevation, e.g. greater and lesser tubercle of humerus, ischial tuberosity, deltoid tuberosity, external occipital protuberance, medial and lateral epicondyles of humerus.21
FIGURE 8: Skeleton
Sometimes they cause linear elevations and the words used are crest, ridge, line, e.g. iliac crest, supracondylar ridge, gluteal lines. Irregular projections are named as processes, spines, e.g. coronoid process on mandible, olecranon process, spine of the scapula. Depressions on the bone are named as pit or fovea, fossa, e.g. pit on the head of the femur, olecranon fossa.
Blood and nerve supply of bones: When you carefully observe the bone you will see number of small foraminae. They are all vascular foramina for the entry of blood vessels and nerves. The biggest of the foramina is the nutrient foramen, generally seen in the middle of a bone. See the nutrient foramen of the femur in the middle of it, and see the vascular foramina near the neck of the femur.
Periosteum: When you try to see the articulated skeleton, it is devoid of periosteum so you will see it rough. But when you observe the bone on the cadaver you will note it to be glistening, it is due to the periosteum. It is the covering on the bone, and helps the Sharpey's fibres to penetrate.22
JOINTS
The area where one or more bones approximate is a joint. The joints are classified based on the tissue that unites them ().
FIBROUS JOINTS: Here the bones are united by fibrous tissue. Go to an articulated skeleton and see how they are articulated. Look at the skull and note that it is made up of number of bones. But the bones look inseparable. These are united by connective tissue and are called sutures.
Sutures: Depending upon, how the bones overlap they are described as simple sutures, e.g. between two maxillae; serrate suture is where the opposing surfaces interlock with serrations or waves, e.g. sagittal suture; dentate suture is where opposing surface look like teeth, e.g. lambdoid suture; squamous suture is where the bones overlap and are united together, e.g. squamous suture. Syndesmosis is where the bones are united by identifiable amount of connective tissue, e.g. lower end of tibia and fibula, interosseous membrane connecting radius and ulna and tibia and fibula. Gomphosis is a specific word used for the teeth fixation into the mandible or maxilla. It is a peg and socket joint.
CARTILAGINOUS JOINT: Here the approximated bones are united together by cartilage.
Primary cartilaginous joint: During development, the bone is laid as a cartilaginous model. Ossification centers develop in this to form the diaphysis (shaft) and the epiphysis (ends). The cartilage which remains between the ossifying centers is hyaline cartilage and is temporary.
FIGURE 9: Joints
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This is a primary cartilaginous joint. In certain places like the costal cartilages it remains throughout life.
Secondary cartilaginous joint/symphysis: Here the cartilage that unites two bones is fibrocartilage. It is present in the midline of the body. It acts like a cushion, e.g. intervertebral disc, symphysis pubis (in articulated skeleton it is represented by corks).
SYNOVIAL JOINT: It is one where the bones are separated and are lubricated by a synovial fluid. The first structure you see in a synovial joint is the capsule and associated ligaments.
Capsule: It is the thick fibrous structure which connects the bones together. The thickness of the capsule is variable. In number of places even muscles are directly inserted into the capsule, e.g. rotator cuff of shoulder joint. Once you open the joint cavity you will feel the sticky substance, the synovial fluid, secreted by the synovial membrane. The synovial membrane is very thin, secretory, and lines the capsule and gets reflected on to the nonarticulating bony surfaces. The synovial fluid prevents friction.
Intra-articular structures: The articular part of the bone appears smooth and glistering as it is covered by the articular cartilage. Number of joints have intra-articular structures within their joint cavities performing different functions. Positioning of intra-articular cartilage separates the joint cavity into two compartments. This results in functional separation of the joint into two joints, and permits two different types of movements, e.g. articular disc in temporomandibular joint. elevation and depression is performed in upper compartment and rotation is performed in lower compartment. Move your jaw and feel it. Presence of fat in the joint cavity fills the incongruities of the bony surfaces, e.g. knee joint. Sometimes tendons pass through the joint. They have a stabilizing effect, e.g. long head of biceps in the shoulder joint. The fat and the tendons in the joint, though lie inside the joint cavity, lie out side the synovial membrane. This prevents the structures from getting damaged during movement.
Blood vessels and nerves are fine branches which enter the joint. They are generally branches derived from the nearby muscular branches. In many a joints the blood vessels form an anastomotic plexuses around the joint and fine branches enter from here to supply the joint, e.g. anastomosis around knee by genicular vessels.
Bursae: Muscles cross the joints closely. So there is every possibility for the muscle to get damaged due to friction. This is prevented by positioning bags of synovial fluid called bursae positioned between the muscle and capsules/bone. In many a places these bursae are continuous with the joint cavity, e.g. knee joint.
CLASSIFICATION OF SYNOVIAL JOINTS: The bones move against each other around an axis, the least mobile line or plane. The shape of the bone determines the type of movement feasible at a joint. Based upon these factors the synovial joints are further classified.
Plane synovial joint—is one without an axis. Look at articulations of carpal bones. You will note that they all have plane surfaces, they slide over each other.
Uniaxial condylar joint—is one where movement is produced around one axis, and the surface of the bone is condylar. Look at the phalanges. See the bulging heads received into the flattened bases. They are like hinges of a door, and are also called hinge joints. Movement takes place around the transverse axis and the movement is flexion and extension.
Uniaxial pivot joint—here the bony surfaces are pivots rotating on a vertical axis. Move your hand to a prone and supine position by holding the radius is your hand. You realize it is moving in a circular fashion.24
Biaxial ellipsoid joint—here the articular surfaces are oval in outline, and produce movement both in transverse and anterior posterior axis. Look at the wrist joint of an articulated skeleton. Perform the movement on your hand. You can perform flexion and extension, and abduction and adduction.
Biaxial sellar joint—here the bones are reciprocally concavo-convex. Movement is possible around two axes. Look at the articulation of trapezium and the base of the first metacarpal bone. Flexion and extension, abduction and adduction are feasible here. Move your thumb and appreciate the movement.
Polyaxial ball and socket joint—here movement is possible in all directions around all possible axes. Look at the articulations at the hip joint. The rounded head of the femur fits into a cup shaped acetabulum. Move the hip joint and see.
NERVES
You will see nerves in every part of your dissection. They are white thread like structures. The nerves conduct impulses from and to different parts of the body.
Even though functionally they carry fibres of different modalities structurally they look the same. The nerves arise from the central nervous system, the brain and the spinal cord. They are named cranial and spinal nerves respectively. Once they leave the central nervous system, the nerves that you see in the dissection belong to peripheral nervous system. The nerves get organized in definitive patterns to reach the different parts of the body.
The cranial nerves (): The nerves you see in the head and neck mainly come from the brain and are cranial nerves. They are twelve in number and supply all the sense organs, the associated musculature and other internal organs.
FIGURE 10: Cranial nerves
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The spinal nerves () are connected to the spinal cord. They are thirty one pairs of them and are segmental in arrangement. They are mixed nerves, they have both motor and sensory parts. The spinal nerves, as soon as they leave the vertebral canal join the blood vessels and form neurovascular bundles. The muscular branches are predominantly motor branches, enter the muscle bellies and supply the muscles. They do have few sensory branches which carry the stretch sensation from the muscles and tendons. The articular branches carry sensations from the joints. The cutaneous nerves are sensory nerves supplying the skin. The named cutaneous nerves pierce the deep fascia, branch to supply the skin. Near the skin they are very fine branches and are difficult to locate. The vascular branches are the fine branches to supply the blood vessels that they accompany.
FIGURE 11: Spinal cord
Nerve plexuses: The upper and lower limbs are outpouchings of the ventral segment of the body. They are supplied by the anterior ramus of the spinal nerves. As they have to pass through narrow passages, they arrange themselves into a plexiform arrangement. In the limbs the nerves have a longitudinal arrangement, e.g. brachial plexus and lumbar plexus. In the trunk region the nerves have a transverse arrangement.
Autonomic nervous system: The internal organs are supplied by the peripheral part of the autonomic nervous system. They are mixed nerves. The sensation that they carry from the internal organs is the stretch sensation. The motor part of the autonomic nervous system has two components, the sympathetic and the parasympathetic. The autonomic motor nerves have relay ganglia in the peripheral nervous systems. For the parasympathetic nervous systems there are called peripheral parasympathetic ganglia. These are located in the head region, e.g. ciliary ganglion, pterygopalatine ganglion. The relay ganglia of the sympathetic nervous system is the sympathetic chain. It is segmental and is paravertebral in position. They supply glands and smooth muscles of organs. The autonomic nervous system forms nerve plexuses, accompany the blood vessels and form neurovascular bundles and enter the organs through the hila, e.g. aortic plexus hepatic plexus ().
FIGURE 12: Autonomic nervous system
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BLOOD VESSELS
The blood vessels form a distribution network in the body. The heart is the organ which controls the blood vascular system (). The aorta begins from the left ventricle of the heart, and gives branches to all parts of the body. These are arteries and carry oxygenated nutritive blood. They end in capillaries at cellular level. The veins begin from the capillaries and carry waste products from the cells and reach the right atrium of the heart as superior and inferior vena cava. The deoxygenated blood thus brought to heart is sent to lungs for purification through pulmonary circulation.
FIGURE 13: Heart
You will see blood vessels throughout your dissection. At the time of death the arteries undergo a wave of contraction. So whatever blood is present in the arteries is pushed into the veins. The blood that gets stagnated, gets solidified there and appears bluish to blackish in color. In a cadaver the fine veins look collapsed but can be easily identified due to this coloration in the veins. The arteries look like thin plastic tubes. They show a bulge and when cut, show a clear cavity within. The big arteries like aorta are accompanied by big veins like inferior vena cava. All the small and medium sized arteries are accompanied by two veins called venae comitantes. In the superficial fascia, they are very fine and are called cutaneous vessels. Near the joints the arteries form alternative anastomic channels. These are fine vessels derived from the nearby blood vessels. This is called collateral circulation/arterioarterial anastomosis (). The blood vessels that reach the brain, heart and retina of the eye are end arteries, they do not anastomose with the neighboring vessels.
FIGURE 14: Collateral circulation/arterioarterial anastomosis
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Functionally the arteries are divided into elastic arteries, muscular arteries and capillary plexus. Elastic arteries are vessels nearest to the heart. They expand when the heart contracts and the blood is pumped into it, e.g. aorta, carotid arteries. Muscular arteries or distribution arteries—these are the medium sized arteries. They control the blood flow to any particular organ as per the need. They have more muscle in their walls, e.g. superior mesenteric, obturator artery. Capillary plexus—the arteries are closed tubes. But when the blood vessels reach the cells they form a capillary plexus. At this level the blood plasma oozes out of the capillaries and forms the tissue fluid, in which each and every cell is bathed. The veins and lymph start from the capillary plexus. You can easily see the capillary plexus, by pressing the fingertip. Note that it becomes red in color. That is the tissue fluid that is seen beneath the skin.
Vascular sheaths: The blood vessels when they pass through narrow passages drag connective tissue sheaths with them. In such circumstances, the sheath over the arteries is thicker and the sheath over the veins is thinner, e.g. axillary sheath. At times the lymphatics occupy a special compartment, e.g. femoral sheath.
The veins begin at the capillary plexus and leave as veins which accompany the arteries. They form venae comitantes around small and medium sized arteries. They are named veins in relation to big arteries, e.g. femoral artery accompanied by femoral vein. Valves are present in all the veins which drain against gravity and those veins which drain towards gravity have no valves, e.g. external jugular vein internal jugular vein. Many a veins form a plexiform arrangement, e.g. pterygoid plexus, pampiniform plexus of veins. In the brain they are located in the thick dura mater and are called venous sinuses. In limbs and head and neck the veins form a superficial and deep system of veins. The superficial set of veins is subcutaneous in location and is not accompanied by arteries, e.g. external jugular vein, great saphenous vein and cephalic vein. These can be easily identified in people who are fair. In them it can be seen as green bulging structures. These are made use of, to inject intravenous fluids and to withdraw blood for diagnostic purposes.
Portal system (): At few places in the body the veins which begin in the capillaries form bigger veins by receiving more tributaries but again break into capillary network. This is a function based arrangement, e.g. the portal vein.
FIGURE 15: Portal system
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This forms in the walls of the gastrointestinal system, drains the nutritive material from here and carries it to the liver for storage where it further breaks down into capillaries. It is called the portal vein. Such portal systems are present in the kidney as well as near the pituitary gland.
Neurovascular bundle: All the structures in the body are supplied by arteries, veins, lymphatics and nerves. They reach the muscles and the organs as a single unit called neurovascular bundle. They are medium sized arteries and are accompanied by two veins called venae comitantes. In a muscle they enter the center of the muscle belly, or they run along the length of the muscle. In an organ they enter the organ at a port called porta or hilum, e.g. hilum of kidney, porta hepatis. In gastrointestinal tract it runs along the length of the organ.
Lymphatics (): Millions and millions of cells in the body are bathed in tissue fluid. The arterial blood gets the nutrition and seeps out into the tissue fluid at the arterial end of the capillary, the waste products get into circulation at the venous end of the capillary. So the tissue fluid is an exudate of the blood. The lymphatic channels begin at the tissue fluid. These channels drain the bigger molecule which cannot enter back into the venous circulation. It is difficult to locate them in the gross anatomy dissection, as they have very thin walls. The lymphatic channels accompany the blood vessels.
FIGURE 16: Lymphatics
Lymph nodes: The lymphatic channels pass through lymph nodes at strategic points in the body. These identify the foreign bodies and initiate the antibody reaction. These are part of immune system in the body. Ultimately, the lymph reaches back into the venous system.
The lymph, the cerebrospinal fluid, the urine and the fluid in the anterior and posterior chamber of the eye, all are filtrates of the blood only. All these form a part of the circulatory system of the body.29
INTERNAL ORGANS
Each of the internal organ has its own size, shape and location. The organs are kept in bags of fluid, e.g. the pericardium, peritoneum. They protect the organs from getting rubbed off during movement. The internal organs are further protected in bony cases, e.g. brain in skull, eyeballs in orbit. The thorax lodges the heart and lungs and the abdomen lodges parts of the gastrointestinal system and urinary system.
 
DISSECTION
While undertaking dissection we follow regional method. We expect one cadaver is given to a batch of students for the academic year. The body is divided into six parts—the upper limb, thorax, abdomen, lower limb, head and neck and central nervous system, and it is dissected in that order.
Upper limb and lower limb: The skeleton with the bones and joints forms the framework of limbs. The muscles which act on the joints are arranged in functional groups and kept in compartments along with its neurovascular bundle. The compartments are separated by septa and all the groups are wrapped by deep fascia and covered by skin.
Thorax and abdomen: Together forms the trunk. Here the skeleton forms an external protective covering. The organs are located in its interior. The organs are kept in specialized compartments, well protected by bags of fluid.
Head and neck: In this region, the head, neck and the back are dealt. The head is the part of communication with the external world. It has a concentration of sense organs and brain and structures associated with them. All these sense organs and the brain are protected in bony cases. The respiratory and digestive systems begin here.
The neck is a passageway for the organs of respiration and digestion which communicate with external world, and the neurovascular bundle where the blood vessels ascend up and the cranial nerves descend down. Back is the part located posterior to the axis, the vertebral column, it is muscular in nature. The vertebral canal lodges the spinal cord.
Central nervous system: It constitutes the brain and spinal cord. It is made up of cell bodies of neurons which form the gray nuclear matter and axons and dendrites which form the tracts. In gross anatomy, it is not possible to identify the individual details. The formalin hardened brain and spinal cord are soft structures. Here we cut the organ with special brain knife and stain with special stain to bring out the differentiation between the nuclei and fibres.30
 
INSTRUMENTS AND THEIR USAGE
The cadaver is your subject and your teacher. The dissection manual and the instruments are your tools to learn this ocean of material. Choose your instruments correctly, use them properly, identify the structures correctly, perform and learn their functions properly ().
FIGURE 17: Instruments
Scalpel with blade: Here two to three sizes are available. You may buy at least two sizes, a bigger size to cut the skin, a smaller blade to cut small structures. The blades may get blunt very fast, so always keep extra blades in hand.
Scissors: Again you need more than one pair of scissors. A 6” blunt and sharp scissors is useful for cutting muscles. A 4” both sharp edge scissors is very much essential to cut smaller structures like tendons, blood vessels and fascia.
Forceps: You need small 4” forceps to hold fine nerves and arteries, a medium 6” plain forceps to do most of the dissection, and a 6” or 8” toothed forceps to hold the skin during skin reflection.
Hook: A single hook is necessary to lift the blood vessels and nerves and a double hook is essential to lift the muscles and organs.
Probe: A long probe or long needles are useful in tracing the fine structures without damaging them.
Chistle, hammer, small saw, brain knife are required for dissections. Generally, these are available in the dissection hall.31
 
DISSECTION METHODOLOGY
Before you begin, you should be familiar with area of dissection. You should read the chapter before hand and be aware of what to expect in your dissection. Make correct incision lines with a white wet chalk piece. Put your first skin reflection line along the skin marking. Use a sharp blade and make a fine cut slowly and gradually through the thickness of the dermis. In skin reflection, you are separating the dermis of the skin from the hypodermis and superficial fascia. You have to use the toothed forceps to pull the skin and hold the blade in a slant and keep putting water as you are reflecting the skin. This will hydrate the connective tissue and make it more yielding. If you are not careful there is more chance of removing the structures in chunks. The structures in the superficial fascia are generally masked by fat. The fat is variable in different parts of the body and in different people. Identifying structures in superficial fascia is time consuming and is not essential to identify all the structures. The deep fascia has many modifications. You should learn to observe all the features before you cut it. Once you cut the deep fascia all the structures, the muscles, vessels, nerves, tendons joints etc. just unfold. All of them are surrounded by the loose connective tissue, the packing material of our body. The loose connective tissue yields when you hydrate it. It is essential to hydrate the parts during dissection. Your responsibility is to clean this loose connective tissue and identify the structures. Once you open the deep fascia do not use the scalpel. Use the forceps, the scissors and the back of the handle to separate the tissue and remove it. Muscles are big and appear brown in color. Use your hand especially while separating the muscles, testing the muscles and holding the bigger blood vessels. Always hold the blood vessels and nerves proximally and use the forceps to clear the connective tissue and trace the branches. While removing the internal organs you need to use the twine and make two knots, cut the organ between the knots. This prevents leakage of the contents. You may use saw, chistle and hammer while removing the brain from the cranial cavity and performing other dissections on the skull.
The dissection schedule of each region is designed in such a way that you penetrate into the body methodically and understand the interrelationship of the structures within a particular area. As you are dissecting, support your study by performing/feeling or locating the structures on a living body. Simultaneously, study the osteology and X-rays/CT scans.