Textbook of Anatomy with Colour Atlas (Volume 1) Inderbir Singh
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1An Elementary Introduction to The Structure of The Human Body

Learning the language of AnatomyChapter 1

 
THE SUBJECT OF ANATOMY
Anatomy is the science that deals with the structure of the human body. Many features of structure can be seen by naked eye and such a study is called gross anatomy or morphological anatomy. Many other features can be observed only under a microscope, and a study of these features constitutes the science of microscopic anatomy or histology. Histology includes the study of details of the structure of cells (cytology), and of related chemical considerations (histochemistry). Many recent advances in our knowledge of the structure of the body have been made possible by the use of high magnifications available with an electron microscope, and such details are referred to as ultrastructure. The science of anatomy also includes the study of the development of tissues and organs before birth: this is called embryology. Aspects of anatomy that are of particular relevance to understanding of disease and its treatment are referred to as applied anatomy or clinical anatomy.
This book deals mainly with gross human anatomy. Brief notes dealing with histology, embryology and clinical anatomy are given where appropriate.
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Fig. 1.1: Scheme to explain the terms anterior, posterior, superior, and inferior.
 
MAIN SUBDIVISIONS OF THE HUMAN BODY
For convenience of description the human body is divided into a number of major parts. Many of the parts bear names with which the student will be already familiar, but even some of these may require more precise definition.
The uppermost part of the body is the head. The face is part of the head (and includes the region of the forehead, the eyes, the nose, the cheeks and the chin). Below the head there is the neck. Note that the junction of the head with the neck is oblique because the neck extends higher up on the back than in front. Below the neck, there is the region that a lay person calls the chest. In anatomical terminology the chest is referred to as the thorax. The thorax is in the form of a bony cage within which the heart and lungs lie. Below the thorax, there is the region commonly referred to as ‘stomach’ or ‘belly’. Its proper name is abdomen. The abdomen contains several organs of vital importance to the body. Traced downwards, the abdomen extends to the hips. A part of the abdomen present in the region of the hips is called the pelvis. The thorax and abdomen together form the trunk. Attached to the trunk there are the upper and lower limbs, or the upper and lower extremities. In relation to the upper limb the terms shoulder, elbow, wrist, hand, palm, fingers and thumb will be familiar. A lay person frequently refers to the entire upper limb as the arm, but in anatomy we use this term only for the region between the shoulder and elbow. The region between the elbow and wrist is the forearm. The fingers and thumb are also called digits.
In the lower limbs the terms hip, knee, ankle, foot and toes will be familiar. The region between the hip and the knee is the thigh, and that between the knee and the ankle is the leg. Like the fingers the toes are also called digits. The innermost, and largest toe, is the great toe.3
 
SOME COMMONLY USED DESCRIPTIVE TERMS
The study of anatomy is like the learning of a new language. Hundreds of new words have to be learnt, and an intimate familiarity with them attained. The learning of these terms is the basic foundation on which all subsequent studies in various subjects of the medical curriculum depend. In short, the study of anatomy teaches us the language of medicine. Of all the terms to be learnt the first, and most fundamental, are those used for precise descriptions of the mutual relationships of various structures within the body. In describing such relationships the lay-person uses terms like ‘in front’, ‘behind’, ‘above’, ‘below’ etc. However, in a study of anatomy, such terms are found to be inadequate; and the student’s first task is to become familiar with the specialized terms used.
A major problem in describing anatomical relationships is that they keep changing with movement. For example, when a person stands upright the head is the uppermost part of the body and the feet the lowermost. However, on lying down the head and feet are at the same level. The problem is overcome by always describing relationships within the body presuming that the person is standing upright, looking directly forwards, with the arms held by the sides of the body, and with the palms facing forwards. This posture is referred to as the anatomical position. We will now consider some descriptive terms one by one.
  1. When structure A lies nearer the front of the body as compared to structure B, A is said to be anterior to B (Fig. 1.1).
    The opposite of anterior is posterior. In the above example, it follows that B is posterior to A. Using these terms we can say that the nose is anterior to the ears; and the ears are posterior to the nose.
  2. When structure C lies nearer the upper end of the body as compared to structure D, C is said to be superior to D (Fig. 1.1). The opposite of superior is inferior. In the above example D is inferior to C. (No difference in the quality of the structures is implied!). Using these terms we can say that the nose is superior to the mouth, but is inferior to the forehead.
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    Fig. 1.2: Scheme to explain the terms medial, lateral and median.
  3. The body can be divided into two equal halves, right and left, by a plane passing vertically through it. The plane separating the two halves is called the median plane (Fig. 1.2). When a structure lies in the median plane it is said to be median in position (e.g., G in Fig. 1.2). When structure E lies nearer the median plane than structure F, E is said to be medial to F. The opposite of medial is lateral. In the above example F is lateral to E.
In the anatomical position the palm faces forwards and the thumb lies along the outer side of the hand. Starting from the side of the thumb (or first digit) the fingers are named index finger (second digit), middle finger (third digit), ring finger (fourth digit) and little finger (fifth digit). To describe the medial-lateral relationships of the fingers we can say that the thumb lies lateral to the index finger. The index finger is medial to the thumb, but is lateral to the middle finger. The middle finger is medial to the index finger and lateral to the ring finger. From these examples it will be clear that the terms anterior-posterior, superior-inferior, and medial-lateral are relative terms. They are not absolute.
Various combinations of the descriptive terms mentioned above are frequently used. For example, each eye is anterior to the corresponding ear; and is also medial to it. Therefore, the eye can be said to be anteromedial to the ear. The tip of the nose is inferior and medial to each eye: we can say the nose is inferomedial to the eye.
We must now consider terms that are sometimes used as equivalent to some of the terms introduced above. The anterior aspect of the body corresponds to the ventral aspect of the body of four-footed animals. Hence the term ventral is often used as equivalent to anterior. (However, we shall see later that the two terms are not always equivalent, e.g. in the thigh). The opposite of ventral is dorsal. In the hand the palm is on the anterior or ventral aspect. This aspect of the hand is often called the palmar aspect. The back of the hand is the dorsal aspect, or simply the dorsum, of the hand. In the case of the foot the surface towards the sole is ventral: it is called the plantar aspect. The upper side of the foot is the dorsum of the foot.
While referring to structures in the trunk the term cranial (= towards the head) is sometimes used instead of superior; and caudal (= towards the tail) in place of inferior. In the limbs the term superior is sometimes replaced by proximal (= nearer) and inferior by distal (= more distant). Using this convention the phalanges of the hands are designated proximal, middle and distal. In the case of the forearm (or hand) the medial side is often referred to as the ulnar side, and the lateral side as the radial side.4
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Fig. 1.3: Scheme showing median and paramedian planes.
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Fig. 1.4: Scheme showing a frontal or coronal plane.
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Fig. 1.5: Scheme showing a horizontal or transverse plane.
Similarly, in the leg (or foot) we can speak of the tibial (= medial) or fibular (= lateral) sides.
In addition to the terms described above there are some terms that are used to define planes passing through the body. (The concept of planes can be understood by reference to a cube. The angles of the cube are points: they have no length. The edges of the cube are lines: they have length but no width. The surfaces of the cube have length as well as breadth, but no thickness: these can be regarded as planes. (Remember than any flat surface is also called a plane surface: the essential thing about a plane is that it is absolutely flat).
We have already seen that a plane passing vertically through the midline of the body, so as to divide the body into right and left halves, is called the median plane. It is also called the mid-sagittal plane. Vertical planes to the right or left of the median plane, and parallel to the latter, are called paramedian or sagittal planes (Fig. 1.3). A vertical plane placed at right angles to the median plane (dividing the body into anterior and posterior parts) is called a coronal plane or a frontal plane (Fig. 1.4). Planes passing horizontally across the body (i.e., at right angles to both the sagittal and coronal planes) and dividing it into upper and lower parts, are called transverse or horizontal planes (Fig. 1.5). In addition there are innumerable oblique planes intermediate between those described above.
Sections through any part of the body in any of the planes mentioned above are given corresponding names. Thus we speak of median sections, sagittal sections, coronal or frontal sections, transverse sections and oblique sections.
 
STRUCTURES CONSTITUTING THE HUMAN BODY
When we dissect up any part of the body we encounter various elements. The basic framework of the body is provided by a large number of bones that collectively form the skeleton. As bones are hard they not only maintain their own shape, but also provide shape to the part of the body within which they lie. In some situations (e.g., the nose or the ear) part of the skeleton is made up, not of bone but of, a firm but flexible tissue called cartilage. Bones meet each other at joints, many of which allow movements to be performed. At joints, bones are united to each other by fibrous bands called ligaments. Overlying (and usually attached to) bones we see muscles. Muscles are what the layman refers to as flesh. In the limbs, muscles form the main bulk. Muscle tissue has the property of being able to shorten in length. In other words muscles can contract, and by contraction they provide power for movements. A typical muscle has two ends one (traditionally) called the origin, and the other called the insertion. Both ends are attached, typically, to bones. The attachment to bone may be a direct one, but quite often the muscle fibres end in cord like structures called tendons which convey the pull of the muscle to bone. Tendons are very strong structures. Sometimes a muscle may end in a flat fibrous membrane. Such a membrane is called an aponeurosis.
When we dissect a limb we find that the muscles within it are separated from skin, and from each other, by a tissue in which fibres are prominent. Such tissue is referred to as fascia.5 Immediately beneath the skin the fibres of the fascia are arranged loosely and this loose tissue is called superficial fascia. Over some parts of the body the superficial fascia may contain considerable amounts of fat. Deep to the superficial fascia the muscles are covered by a much better formed and stronger membrane. This membrane is the deep fascia. In the limbs, and in the neck, the deep fascia encloses deeper structures like a tight sleeve. Membranes similar to deep fascia may also intervene between adjacent muscles forming intermuscular septa. Such septa often give attachment to muscle fibres.
Running through the intervals between muscles (usually in relation to fascial septa) there are blood vessels, lymphatic vessels, and nerves.
Blood vessels are tubular structures through which blood circulates. The vessels that carry blood from the heart to various tissues are called arteries. Those vessels that return this blood to the heart are called veins. Within tissues arteries and veins are connected by plexuses of microscopic vessels called capillaries.
Lymphatic vessels are delicate, thin walled tubes. They are difficult to see. They often run alongside veins. Along the course of these lymphatic vessels small bean shaped structures are present in certain situations. These are lymph nodes. Lymphatic vessels and lymph nodes are part of a system that plays a prominent role in protecting the body in various ways that you will study later.
Running through tissues, often in the company of blood vessels, we have solid cord like structures called nerves. Each nerve is a bundle of a large number of nerve fibres. Each nerve fibre is a process arising from a nerve cell (or neuron). Most nerve cells are located in the brain and in the spinal cord. Nerves transmit impulses from the brain and spinal cord to various tissues. They also carry information from tissues to the brain. Impulses passing through nerves are responsible for contraction of muscle, and for secretions by glands. Sensations like touch, pain, sight and hearing are all dependent on nerve impulses travelling through nerve fibres.
Bones, muscles, blood vessels, nerves etc., which we have spoken of in the previous paragraphs are to be seen in all parts of the body. In addition to these many parts of the body have specialized organs, also commonly called viscera. Some of the viscera are solid (e.g., the liver, or the kidney), while others are tubular (e.g., the intestines) or sac like (e.g., the stomach). The viscera are grouped together in accordance with function to form various organ systems. Some examples of organ systems are the respiratory system responsible for providing the body with oxygen; the alimentary or digestive system responsible for the digestion and absorption of food; the urinary system responsible for removal of waste products from the body through urine; and the genital system which contains organs concerned with reproduction.
From the discussions in the previous paragraphs it will be clear that in the study of the anatomy of any part of the body we have to consider the following:
  1. The skeletal basis of the part including bones and joints.
  2. The muscles and fasciae.
  3. The blood vessels and nerves.
  4. The lymph nodes and their areas of drainage.
  5. Viscera present in the region.
 
REGIONAL AND SYSTEMIC STUDY OF ANATOMY
The best method for the study of gross anatomy is by dissection of a cadaver. In doing so we naturally study all structures of a region together, and obtain a good idea of their mutual relationships. This kind of study is called regional anatomy.
The major disadvantage of a regional approach to the study of anatomy is that in any given region we often see only small segments of structures that may have a much wider distribution. This is specially true in the case of blood vessels and nerves. It is because of this fact that many text books of anatomy follow what is called the systemic approach. In studying (for example) the arteries in this way we begin with the main artery leaving the heart and trace its branches into all parts of the body.
It will be obvious that both the regional and systemic approaches have some advantages. However, students doing their dissections find it more convenient if all facts relevant to the region of study are available at one place. In this book we will, therefore, follow the regional approach, but will supplement it where necessary by a systemic review of the distribution of blood vessels and nerves within the part.
By its very nature the study of anatomy by dissection is a time consuming process. Because of this a student who enters a medical college may find that at the end of a year of study he or she may know very little about the anatomy of some very important organs that belong to regions not yet dissected. Meanwhile, teachers in other subjects could be making repeated references to such organs. This is an unsatisfactory situation. To help students to cope with this problem, to some extent, this book begins by reviewing elementary features of the anatomy of different parts of the body and of different organ systems. All these will be studied in detail in later sections.