Having studied the techniques of dissection, it will be useful to you to note some of the structures that you will encounter during dissection. In dissecting the human body you will come across various structures such as skin, superficial fascia, deep fascia, muscles, tendons, blood vessels, lymph vessels and nodes, nerves, bones, joints and organs.

Skin

The skin forms the outer covering of the body and is composed of a superficial layer, the epidermis, and a deep layer, the dermis. The skin is

slightly thicker over the extensor than over the flexor surface. However, it is extremely thick over the palms and soles, which are, in fact flexor surfaces.

Superficial fascia

The superficial fascia is deep within the skin. Within this layer lie the superficial nerves, blood vessels and lymphatics. The superficial fascia consists of loose areolar tissue and is filled with fat. In some situations like the anterior abdominal wall and gluteal region (buttock), there is a large accumulation of fat. Since fat is an insulator, the superficial fascia acts as retainer of body heat.

Deep fascia

Deep to the superficial fascia lies the deep fascia. This is a tough connective tissue tunic which covers the underlying muscles. Sometimes the muscles are attached to this fascia. The deep fascia also sends in septa between muscles, providing a covering for them as well as sheaths for blood vessels and nerves. Occasionally the deep fascia sends extensions between different functional groups of muscles. These extensions that gain attachments to bones are called intermuscular septa. The deep fascia is also extremely thickened in the region of the wrist and ankle where they form well defined transverse bands extending across bony prominences. These bands are called the retinacula. Along with the underlying bone, they form osteofascial tunnels providing a passage for tendons, which are thus prevented from springing out during contraction of the muscles.

Muscle

When the deep fascia has been cleared away you will reach the muscle. This is commonly known as voluntary, skeletal or striated muscle. The muscles contribute to about 50 per cent of the body weight and are composed of muscle fibres. The fibres in individual muscles are arranged in different ways so that the muscles are often described as strap-like when the individual fibres are long and arranged in parallel; fusiform when a fleshy belly tapers towards both ends, often ending in a tendon; and pennate when there is a resemblance to a feather. Pennate muscles are described as unipennate, bipennate or multipennate. These various types will be encountered during dissection.

Muscles are usually connected at their ends to skeletal elements. These attachments of a muscle are usually described as its origin and insertion. The origin is generally the proximal attachment and is usually the fixed point from which the muscle acts so that the skeletal element into which it is inserted distally is able to move. Usually the origins of muscles are fleshy and their insertions tendinous. The tendons are formed by condensations of fibrous tissue and possess great tensile strength. Sometimes a tendon may be flat and thin, and when it forms a broad sheet, it is called an aponeurosis. Contraction of muscles in the living can be seen and felt. This can be tested by making a muscle contract against resistance. Muscles have a rich blood and nerve supply. The point of entry of these into a muscle is called the neurovascular hilus. The innervation of muscles is both motor and sensory. The stimulation of the motor nerve causes a contraction of the muscle. The sensory nerve carries information about the nature of the force of contraction, degree of stretch, etc. of the muscle it innervates.

Peripheral Nerves

Knowledge of the anatomy of a nerve must include its origin, course, relations, distribution and surface anatomy.

Origin

Many peripheral nerves arise as branches of a larger nerve trunk, but others arise directly from a plexus. A knowledge of the segments of the spinal cord from which the motor fibres in the nerve arise and to which its sensory fibres are conveyed is important. This root value of the nerve is of value in diagnosing the site of damage to the peripheral or central nervous system.

Course and relations

A knowledge of the pathway followed by the nerve from its origin to its termination is essential to enable you to find it in case of an injury, to avoid it when making injections, or to deduce possible damage to it from a wound or fracture. The relations of a nerve must be given in a logical manner. Nerves are usually suitable key structures around which to build your knowledge of a region.

Distribution

This involves a knowledge of the muscles and skin supplied by the nerve. The autonomic fibres running in the nerve also supply blood vessels, sweat glands and the muscles of the hairs. Nor are the sensory fibres restricted to the skin; they also supply sensory fibres to the deeper tissues, including the muscles. A degree of sensory overlap exists with neighbouring nerve distributions. This is sometimes so great that very little noticeable change is produced by cutting a single nerve. An account has to be taken of the effects produced by cutting a nerve. There is first a motor paralysis, which will result in certain specific movements becoming weakened or lost altogether. The paralysed muscles are flaccid and cannot resist the pull of their healthy antagonists; the joint upon which they act may thus be pulled into a position other than its normal position of rest, producing a postural deformity. The wasting of the muscles concerned will also produce a configurational deformity, obvious when the affected region is compared with its healthy counterpart. The tendon reflexes which depend upon the integrity of the nerve will be lost, and the absence of the motor sympathetic fibres leads to vascular disturbances and an absence of sweating in the affected territory. On the sensory side, the autonomous zone of the nerve will be completely anaesthetic, and the overlap zones manifest themselves as an annulus of partial sensibility surrounding the autonomous zone. The loss of deep sensibility may be of great importance.

Surface anatomy

It is important to be able to map out the course of a nerve, and to outline its cutaneous distribution and elicit the deep and superficial reflexes which it serves. One must know the rough distribution in the skin of each spinal segment; these segmental distributions are known as dermatomes and are an aid to diagnosing the level of spinal injury. The common and simple tests which can be used to tests the integrity of the nerve have to be learned. After an accident, it is often alleged that subsequent paralysis is due to the faulty treatment rather than to the injury itself.

Blood vessels

Blood vessels are of three major types arteries, veins and capillaries. Arteries are thick-walled vessels which carry blood away from the heart to the tissues. Veins return the blood from the tissue to the heart. Intervening between the arterial and venous sides of the circulation are minutes vessels called capillaries. In the cadaver, the arteries appear paler and are palpably thicker, while the veins are bluish or dark in colour with thin walls. Veins superficial to the deep fascia often run alone while those deep to the deep fascia accompany arteries and are called venae comitantes. In the living, arteries are pulsatile and their pulsations are visible when they lie close to the surface. Such arteries are usually palpated against an underlying bone, e.g., pulsations of the radial artery are felt by compressing it against the radius (a pressure point).

Lymphatics

The cells comprising the tissue of the body are bathed in fluid called tissue fluid, which is derived from blood plasma. Tissue fluid provides a medium for transport of nutrients to cells as well as for removal of their waste products. Part of the tissue fluid re-enters the blood circulation but the remainder (lymph) is drained by a system of extremely thin walled channels called lymph capillaries. They unite to form lymph channels which accompany blood vessels. The lymphatic channels lying superficial to the deep fascia generally accompany the superficial veins, while the deeper vessels accompany arteries. Lymph vessels are not normally seen during dissection. During their course they are interrupted by lymph nodes, containing discrete aggregations of lymphocytes which are a type of white blood cells. Tissue fluid in the lymph channels after filtering through lymph nodes is carried by lymphatic channels of increasing calibre which ultimately enter the large veins in the neck.

Bones

Bones form the major part of the human skeleton and provide the supporting framework for the body. Although they appear to be rigid, they are in fact extremely plastic. During growth and repair this plasticity is easily seen. Even at normal times there is a continuous turnover of the constituents of bone. Bone is in fact an organ and not merely a tissue since it has in its matrix, nerves, blood vessels and lymphatics like any other organ of the body. Bones are generally classified according to their shapes. Long and short bones are peculiar to the limbs; flat bones are generally found in the girdles of the limbs, ribs and vault of the skull; irregular bones are peculiar to the vertebral column and base of the skull. Sesamoid bones are those that are developed in some tendons; the knee cap or patella is a good example of a sesamoid bone. Each long bone has a shaft or diaphysis, and two ends or epiphyses. The diaphysis and the epiphysis are developed from separate ossification centres. The ossification of the diaphysis invariably begins before birth, whereas the centres for the epiphyses usually form after birth. The epiphyses unite with the diaphysis at different times. The epiphysis which begins ossifying first usually unites with the diaphysis later. Since this end of the diaphysis continues to grow in length after the opposite end has ceased its growth, it is called the growing end of the bone. The shaft of a typical long bone usually presents a prominent foramen somewhere about its middle. This is called the nutrient foramen as it transmits fairly large blood vessels called the nutrient vessels which supply the shaft. The canal for the nutrient artery is invariably directed away from the growing end of the bone. Most of the bony surfaces provide attachments for muscles. Fleshy attachments of muscles usually leave no marks on the bone. Tendinous attachments if flattened or aponeurotic leave rough markings. Thick tendons are usually attached to smooth areas which are either depressed or raised. Admixture of tendon and muscle fibres invariably leave very rough markings on bones.

Joints

Junctional regions between bones develop into joints. Joints can be classified as fibrous, cartilaginous or synovial, depending on the type of tissue present between the articular ends of the bones. Generally, fibrous joints permit very little movement, while synovial joints provide the greatest degree of freedom of movement. The cartilaginous joints form an intermediate group. However, there are exceptions to these general statements.