• It has a superficial thin layer of compact bone around a central mass of spongy bone, and contain internal soft tissue, the marrow, where blood cells are formed.
• It serve as a reservoir for calcium and phosphorus and act as biomechanical levers on which muscles act to produce the movements permitted by joints.

• Diaphysis

  • Forms the shaft (central region) and is composed of a thick tube of compact bone that encloses the marrow cavity.
• Metaphysis

  • Is a part of the diaphysis, the growth zone between the diaphysis and epiphysis during bone development.
• Epiphyses

  • Are expanded articular ends, separated from the shaft by the epiphyseal plate during bone growth and composed of a spongy bone surrounded by a thin layer of compact bone.

 

• Ossification is the process of laying down new bone material by cells called osteoblasts. It is of two types:

  • Membranous ossification is the direct laying down of bone into the mesenchyme (embryonic connective tissue).
  • Endochondral ossification involves osteogenesis in a precursor model of cartilage.
• Membrane (dermal) bones ossify in membrane (intramembranous ossification), and are thus derived from mesenchymal condensations. The flat bones of the skull and face, the mandible, and the clavicle develop by intramembranous ossification.
• Cartilaginous bones ossify in cartilage (endochondral ossification), and are thus derived from preformed cartilaginous models. The bones of the extremities (limbs) and those parts of the axial skeleton that bear weight (vertebral column and thoracic cage) develop by endochondral ossification.
• Membrano-cartilaginous bones are initially formed in membrane but later partly in cartilage. Examples: clavicle, mandible, occipital, temporal, sphenoid.
• Cartilaginous ossification involves primary and secondary centers of ossification:
• Primary center of ossification

  • In long bones, bone tissue first appears in the diaphysis (middle of shaft).
  • Primary centers starts appearing at week 6 of intrauterine life.
  • Chondrocytes multiply and form trabeculae and cartilage is progressively eroded and replaced by bone, extending towards the epiphysis.
  • A perichondrium layer surrounding the cartilage forms the periosteum, which generates osteogenic cells that make a collar to encircles the exterior of the bone and remodels the medullary cavity on the inside.
  • The nutrient artery enters via the nutrient foramen from a small opening in the diaphysis.

    • It invades the primary center of ossification, bringing osteogenic cells (osteoblasts on the outside, osteoclasts on the inside.)
    • The canal of the nutrient foramen is directed away from more active end of bone when one end grows more than the other.
    • When bone grows at same rate at both ends, the nutrient artery is perpendicular to the bone.
• 2Secondary center of ossification

  • The secondary centers generally appear at the ends (epiphysis) of long bones.
  • Secondary ossification mostly occurs after birth except for secondary centers around knee joint (distal femur and proximal tibia), which appear during last weeks of fetal life (or immediately after birth).
  • The epiphyseal arteries and osteogenic cells invade the epiphysis, depositing osteoblasts and osteoclasts which erode the cartilage and build bone. This occurs at both ends of long bones but only one end of digits and ribs.

 

• The growing ends of bones in upper limb are upper end of humerus and lower ends of radius and ulna.
• In lower limb, the lower end of femur and upper end of tibia are the growing ends.
• The nutrient foramen is directed away from the growing end of the bone; their directions are indicated by a memory aid: ‘Towards the elbow I go, from the knee I flee’.

 

• Each branch divides into a number of small parallel channels which terminate in the adult metaphysis by anastomosing with the epiphysial, metaphysial and periosteal arteries.
• It supplies medullary cavity, inner 2/3 of cortex and metaphysis.
• The nutrient foramen is directed away from the growing end of the bone. Memory aid: Towards the elbow I go, from the knee I flee.
• 3Bony skeleton is divided into two parts: the axial skeleton and the appendicular skeleton.

  • Axial skeleton is the central core unit, consisting of the skull, vertebrae, ribs, and sternum.
  • Appendicular skeleton comprises the bones of the extremities.

 

• They are commonly found where tendons cross the ends of long bones in the limbs.
• Sites of sesamoid bones:

  • In the ear: The lenticular process of incus is a sesamoid bone and therefore is considered the fourth ossicle of middle ear.
  • In the hand: Two sesamoid bones in the distal portions of the first metacarpal bone (within the tendons of adductor pollicis and flexor pollicis brevis).
  • In the wrist: The pisiform of the wrist is a sesamoid bone (within the tendon of flexor carpi ulnaris), develops at age 9–12.
  • In the knee: The patella (within the quadriceps tendon).
  • Fabella in the lateral head of gastrocnemius behind the knee joint.
  • Sesamoid bone in the tendon of peroneus longus where it binds around the cuboid bone.
  • In the foot: Two sesamoid bones in the distal portions of the first metatarsal bone (within the tendons of flexor hallucis brevis.

 

• They are generally produced during development by excavation of bone by pneumatic diverticula (air sacs) from an air-filled space such as the nasal cavity.
• E.g., maxilla, frontal, sphenoid, and ethmoid bones and a part of the mastoid part of the temporal bone.

 

• Pressure epiphysis are the parts of bone involved in weight transmission (and are intracapsular), e.g. head of humerus and femur and condyles of humerus, femur, tibia, etc.

• Traction epiphysis are present at the ends of bones and develop due to traction by the attached muscles (and are therefore extracapsular), e.g. greater (and lesser) tubercles in humerus and greater (and lesser) trochanter in femur.

  • These epiphyses ossify later than pressure epiphyses.
  • Examples of traction epiphyses are tubercles of the humerus (greater tubercle and lesser tubercle), and trochanters of the femur (greater and lesser). Mastoid process is also a traction epiphysis.
• 4Atavistic epiphysis: These types of fused bones are called atavistic, another example is the coracoid process of the scapula, posterior tubercle of talus (as trigonum), which has been fused in humans with the main bone, but is separate in lower animals.
• Aberrant (unusual) epiphysis are deviations from the norm and are not always present. E.g., Epiphyses at the head of the first metacarpal and at the base of other metacarpals.

Questions Answers    

Synarthrosis (immovable)	Fibrous joints
Amphiarthrosis (slight mobile)	Cartilaginous joint
Diarthrosis (freely mobile)	Synovial joints

Types of fibrous joint	Examples
Suture	Spheno-vomerine joint (schindylesis)
Gomphosis	Tooth and socket joints
Syndesmosis	Middle radioulnar joint Inferior radioulnar joint

• Primary Cartilaginous Joints (synchondroses) are united by hyaline cartilage and permit no movement but growth in the length.

  • A primary cartilaginous joint (synchondrosis) is one where bone and hyaline cartilage meet. Thus, all epiphyses are primary cartilaginous joints, as are the junctions of ribs with their own costal cartilages.
  • All primary cartilaginous joints are quite immobile and are very strong. The adjacent bone may fracture, but the bone–cartilage interface will not separate.
  • They Includes epiphyseal cartilage plates (the union between the epiphysis and the diaphysis of a growing bone) and spheno-occipital and manubrio-sternal synchondroses.
• Secondary cartilaginous joints (Symphysis) have bones are united by hyaline plus fibrocartilage.

  • These joints are usually in the midline and are slightly mobile.
  • Include pubis symphysis, midline intervertebral joints.

    • Symphysis is a union between bones whose articular surfaces are covered with a thin lamina of hyaline cartilage. The hyaline laminae are united by fibrocartilage.
    • There may be a cavity in the fibrocartilage, but it is never lined with synovial membrane and it contains only tissue fluid.
    • Examples are the pubic symphysis and the joint of the sternal angle (between the manubrium and the body of the sternum).
    • An intervertebral disc is part of a secondary cartilaginous joint, but here the cavity in the fibrocartilage contains a gel.
    • A limited amount of movement is possible in secondary cartilaginous joints, depending on the amount of fibrous tissue within them. All symphyses occur in the midline of the body.

Types of cartilaginous joint	Examples
Synchondrosis	Spheno-occipital joint Epiphysio-diaphyseal joint (growing bone)
Symphysis	Midline intervertebral joint Sacrococcygeal joint

• Synovial joints are freely mobile joints.
• Synovial joints are uniaxial: Plane, hinge and pivot; Biaxial: Condylar and ellipsoid; Multiaxial: Saddle, ball and socket.

 

Types of synovial joint	Examples
Plane	Acromioclavicular Intercarpal Intertarsal
Hinge	Elbow Interphalangeal
Pivot (Trochoid)	Atlanto-axial Superior radio-ulnar Inferior radio-ulnar
Condylar	Temporo-mandibular Knee joint

Types of synovial joint	Examples
Ellipsoid	Atlanto-occipital Wrist (radio-carpal) Metacarpo-phalangeal (knuckle)
Saddle	Malleus-incus joint Sternoclavicular First carpo-metacarpal Calcaneocuboid
Ball and socket	Incus-stapes joint Shoulder Hip Talo-calcaneo-navicular

• Knee joint is a complex joint (involving more than two bones).
• Femoro-tibial joint structurally resembles a hinge joint, but is considered as a condylar type of synovial joint between two condyles of the femur and tibia. In addition, it includes a saddle joint between the femur and the patella.

Questions Special AIIMS Pattern Question Answers Special AIIMS Pattern Answer  

• Muscles with parallel fasciculi: These are muscles in which the fasciculi are parallel to the line of pull and have greater degree of movement. These muscles may be:

  • Quadrilateral, e.g. thyrohyoid, Pronator quadratus
  • Strap-like, e.g. sternohyoid and sartorius
  • Strap-like, with tendinous intersections, e.g. rectus abdominis
  • Fusiform, e.g. biceps brachii, digastric
• Muscles with oblique fasciculi: When the fasciculi are oblique to the line of pull, the muscle may be triangular, or pennate (feather-like) in the construction. This arrangement makes the muscle more powerful, although the range of movement is reduced. Oblique arrangements are of the following types:
• Convergent fasciculi: The muscle fibers or fasciculi converge at the insertion point to maximize contraction. Such muscles may be: (a) Triangular, e.g. temporalis (b) Fan-shaped, e.g. temporalis.

• 10Spiral or twisted fasciculi: In some muscles the fibers are twisted or spiraled, e.g. trapezius, latissimus dorsi, pectoralis major, supinator.
• Cruciate muscles: In some muscles, fibers or fasciculi are arranged in superficial and deep planes and crossed ‘X’, e.g. sterno-cleidomastoid, masseter, adductor magnus.
• Sphincteric fasciculi: In some muscles, the muscle fibers or fasciculi surround an opening or orifice, thus when they contract the opening is closed or constricted, e.g. orbicularis oculi around the eye and orbicularis oris surrounding the oral orifice.
• Pennate fasciculi: The pennate-fiber muscles resemble the feather, the fleshy fibers correspond to the bars of the feather and the tendon to the shaft, as they are all inserted by tendon. They may be:

  • Uni pennate, e.g. flexor pollicis longus, extensor digitorum longus
  • Bipennate, e.g. rectus femoris, flexor hallucis longus
  • Multipennate, e.g. tibialis anterior, subscapularis, deltoid (acromial fibers).
• Common sites of intramuscular injection

Upper arm (Deltoid)	5 cm distal to the acromion or 4 cm proximal to the insertion of deltoid This is to prevent injury to circumflex humeral nerve.
Gluteal region	Upper outer (superolateral) quadrant This is to avoid damage to superior and inferior gluteal vessels and sciatic nerve. The muscle in which the injection is given is gluteus medius.
Thigh (lateral aspect)(vastus lateralis)	Infant: Upper lateral quadrant of thigh below GT Adult: Middle third of lateral aspect.

• They lie at the boundaries between muscle groups. Usually it incorporates fibers from two adjoining groups and is therefore supplied by two nerves usually.

Muscle	Nerve supply (Part of muscle)
Trapezius	Spinal accessory nerve (motor) Ventral rami of C3, C4 (proprioception)
Digastric	Trigeminal nerve (anterior belly) Facial nerve (posterior belly)
Pectoralis major	Medial pectoral nerve Lateral pectoral nerve
Subscapularis	Upper subscapular nerve Lower subscapular nerve
Brachialis	Musculocutaneous nerve (motor) Radial nerve (proprioceptive)
Flexor digitorum profundus	Median nerve (lateral half) Ulnar nerve (medial half)
Opponens pollicis	Median nerve (superficial part) Ulnar nerve (deep part)
Ilio-psoas	Direct branches of the anterior rami of L1-L3 (psoas major) Femoral nerve (iliacus)
Pectineus	Femoral nerve (anterior fibers) Obturator nerve (posterior fibers)
Biceps femoris	Tibial part of sciatic nerve (long head) Common peroneal nerve (short head)
Adductor magnus	Tibial part of sciatic nerve (ischial part) Obturator nerve (adductor part)

Questions Answers  

• Portal circulation is a capillary network that lies between two veins. Blood supplying the organ thus passes through two sets of capillaries before it returns to the heart.
• In hepatic portal system blood supplying the abdominal organs passes through two sets of capillaries before it returns to the heart.
• A portal circulation also connects the median eminence and infundibulum of the hypothalamus with the adenohypophysis.
• In the renal glomeruli. The glomerular capillary bed lies between afferent and efferent arterioles and may be considered as a portal circulation, but most of the authors do not mention so (including Gray's anatomy).

  Questions Answers