Terminology

For the purpose of description the body is considered to be in the anatomical position. In this position the subject is assumed to be standing, the feet together, the arms to the side, and with the head and eyes and the palms of the hands facing forwards. To ensure consistency of description, it is important to keep the anatomical position constantly in mind. The position of structures relative to each other in the body is defined in relation to the following planes: The Median Plane: This is the back-to-front vertical plane which cuts through the body in the midline. This plane bisects the body into symmetrical right and left halves. A Sagittal Plane: This is any vertical plane parallel to the median plane. A Frontal Plane or Coronal Plane: This is any vertical plane at right angles to the median plane. A Transverse Plane: This is any horizontal plane through the body at right angles to both the sagittal and frontal planes. Any structure lying closer than another to the midline of the body is said to be medial to it, and any further from the midline lateral. Every structure automatically has a medial and a lateral aspect. A point or plane in space closer than another to the head-end of the body is said to be superior to it, and, conversely, the point or plane further away is inferior. The terms cranial and caudal replace the terms ‘superior’ and ‘inferior’ in descriptions of the embryo, and they are also sometimes replaced by the terms rostral and caudal in descriptions of the brain. The terms proximal and distal are used in describing parts of the limbs which are closer to or further from the attachment of the limbs to the trunk. The front surface of the body, or of any structure in the body, is called its anterior surface, and conversely the back of any surface is denoted by the term posterior. The terms ventral and dorsal are synonymous with ‘anterior’ and ‘posterior’. The term supine refers to the body lying on its back, i.e. dorsal surface. The term prone refers to the body lying on its face, i.e. ventral surface. The hand is said to be supinated when the palmar surface faces forwards as in the anatomical position. When the hand is rotated so that the palmar surface faces posteriorly, it is said to be pronated. The sole of the foot is known as the plantar surface. When the plantar surface is turned medially, the foot is inverted; when laterally, everted. Structures that lie near to the surface of the body are described as superficial to others that lie on a deep plane. The term external describes structures outside an area, space, or structure, and the term internal describes those within. When referring to structures of the wrist and hand, the terms radial and ulnar are often used instead of ‘lateral’ and ‘medial’. This avoids any confusion due to the fact that when the hand is pronated, its lateral border (i.e. the side of the thumb) lies ‘medial’ to the side of the little finger, which in the supinated position is medial. And in the leg and foot, the terms tibial and fibular are often used instead of ‘medial’ and ‘lateral’. The term flexor surface generally refers to the ventral aspect of the body while the dorsal aspect is referred to as the extensor surface; but the lower limb is an exception in that the extensor surface has become ventral, owing to the fact that it has undergone rotation during fetal life. The terms pre-axial and post-axial borders are used in reference to the margins of the limbs. The pre-axial border is in relation to the thumb (pollux) and first toe (hallux) while the post-axial border is in relation to the little finger and fifth toe.

UPPER LIMB

Human beings belong to the Order Primates. This group of mammals are more renowned for their adaptability than for their adaptation, thus they have avoided the pitfalls of specialisation and consequently have become the most flourishing group of mammals during the past 50 to 60 million years. Most primates are arboreal. This arboreal habitat has not only given them security from enemies, but also has improved their sense of vision. With this improvement in visual perception there has been a corresponding reduction of their olfactory sense organs, the latter having become unimportant in an arboreal environment. This reduction of the olfactory apparatus has been accompanied by a recession of the snout which is the region of exquisite tactile sensibility in macrosmatic animals. In the human species, this tactile function is taken over chiefly by the upper limbs, which have become emancipated from the burden of weight-bearing. Such tactile sensibility has become specially marked towards the distal parts of the upper limb, i.e. digits. Moreover, humans have also inherited the prehensile or grasping function of the upper limbs from their arboreal ancestry. Superimposed on to the more primitive prehensile function of the hand is the evolution of finer precision movements. Thus the hand has developed into a prehensile organ possessing strength, stability, mobility, a high degree of neuromuscular coordination and sensory discrimination. This, together with the specialised development of the human brain, i.e. the combination of the human hand and mind, has culminated in the manufacture and use of tools, thus giving birth to the rich human cultural activities of arts and crafts. Consequently the human hand is both a sensory and a motor apparatus. One also has to be aware that although over the past 50 million years there has been little change in the structure of the skeleton and muscles of the hand, the changes in function have been profound. For example, the opposability of the thumb to the index finger is a unique human trait and it is important to realise that functionally the thumb represents one half of the hand. And in connection with the functional aspect, the clinical importance of the hand is obvious since 45 per cent of all industrial injuries are associated with the hand. In the study of the upper limb as well as other parts of the body, you must bear in mind the functional correlates of the morphological structures that you encounter during the dissection sessions.

SUPERFICIAL DISSECTION OF BACK OF TRUNK, SCAPULAR REGION AND BACK OF ARM

1. With the body in the prone position make the following incisions:

(a)a midline incision from about the spine of T1 down to the tip of the coccyx;

(b) from the tip of the coccyx curving upwards and laterally along the iliac crest to the mid-axillary line (if not already done);

(c) from the spine of L1 vertebra laterally to the midaxillary line;

(d) from the spine of T1 laterally to about 2 cm above the lateral end of the clavicle; and

(e) cut transversely across the middle of the back of the forearm to its medial border. Reflect the skin flaps.

2. At the back of the neck and trunk, note the posterior rami of the cervical, thoracic and lumbar nerves, which may be accompanied by small vessels. These will be seen piercing the deep fascia about 3–4 cm from the midline.

3. Remove the superficial and deep fasciae to expose the first layer of muscles of the back of the trunk i.e. trapezius and latissimus dorsi. Note the origin of the trapezius from the occipital bone to the spine of the twelfth thoracic vertebra, and its insertion into the clavicle, acromion process and the crest of the spine of the scapula. Similarly define the origin of the latissimus dorsi from the lower thoracic spines, thoracolumbar fascia, iliac crest, lower ribs and inferior angle of the scapula, and its insertion into the intertubercular groove of the humerus.

4. Divide the trapezius vertically about 2 cm from the midline working from below upwards to the level of the horizontal skin incision

(d) . Clean and preserve the descending accessory nerve entering the deep surface of the lateral portion of the divided muscle. Then cut the trapezius horizontally along the skin incision

(d) and from its attachment to the clavicle and acromion and turn the lower cut part of the trapezius laterally.

5. Note the second layer of muscles lying deep to the trapezius. They are from above downwards

(a)levator scapulae,

(b) rhomboid minor, and

(c) rhomboid major. They are inserted serially along the medial border of the scapula above, at and below the spine. Divide the three muscles close to the medial border of the scapula.

6. Clean the deltoid muscle arising from the anterior aspect of the lateral third of the clavicle, lateral border of the acromion and the crest of the spine of the scapula. Note the insertion of the muscle into the deltoid tuberosity of the humerus. Examine the direction of the anterior, middle and posterior fibres of the muscle.

7. Cut the deltoid muscle close to its origin and push the muscle downwards and secure the axillary nerve lying on its deep surface. It supplies the deltoid and teres minor muscles.

8. Identify the subacromial bursa situated partly below the acromion process and partly below the coracoacromial ligament (arch). Deep to the bursa is the supraspinatus tendon.

9. Clean the supraspinatus and infraspinatus muscles arising from the supra- and infraspinous fossae of the scapula. They are inserted into the upper and middle facets on the greater tubercle of the humerus. These two muscles are supplied by the suprascapular nerve.

10. Divide the latissimus dorsi vertically below the inferior angle of the scapula and separate the two parts of the muscle. The three muscles attached along the axillary (lateral) border of the scapula can now be seen. These are from below upwards: the teres major, teres minor and long head of triceps.

1. Feel the origin of the subscapularis muscle on the anterior surface of the scapula and its insertion into the lesser tubercle of the humerus. Quickly review the insertions of the supraspinatus, infraspinatus, teres minor and subscapularis tendons. Note their intimate relationship to the capsule of the shoulder joint thus forming the rotator cuff.

2. Examine the slips of origin of serratus anterior passing from the upper eight ribs to the whole length of the medial border of the scapula, the last four slips passing to the inferior angle. Divide the serratus anterior vertically along the medial border of the scapula.

3. Cut though the middle of the clavicle with a saw.

4. Next remove the upper limb by cutting the remaining structures close to the first rib. The upper limb is now free.

Back of Arm

5. In order to examine the radial nerve which gives branches to the long, lateral and medial heads of the triceps muscle, cut the long head of triceps close to its origin from the infraglenoid tubercle of the scapula and displace it medially. Follow the radial nerve distally between the lateral and medial heads of the triceps where it enters the groove for the radial nerve on the posterior surface of the humerus. Detach the lateral head of triceps from its origin from the upper part of the posterior surface of the humerus. The medial head arises from the posterior surface of the humerus below the groove. Note that the radial nerve supplies also the anconeus muscle, which is situated behind and lateral to the elbow joint.

6. Define the common insertion of the triceps on the olecranon process of the ulna.

7. Follow the radial nerve from the groove for the radial nerve towards the lateral side of the arm where it pierces the lateral intermuscular septum. Observe that the nerve subsequently lies on the brachialis and is overlapped by the brachioradialis and extensor carpi radialis longus. The radial nerve supplies all three muscles. Observe the division of the radial nerve into superficial and deep branches just above the elbow.

Summary

The muscles of the back of the trunk are arranged in layers. The superficial two layers are chiefly concerned in attaching the upper limb girdle to the trunk. Consequently the majority of the muscles such as the trapezius, rhomboids and levator scapulae act only on the shoulder girdle. However, the latissimus dorsi, which passes from the trunk to the humerus, has actions both on the girdle and the shoulder joint. The different parts of the trapezius may act together or independently. Thus the upper fibres of the muscle may act independently in shrugging the shoulder, or they may act in concert with the lower fibres as in rotating the scapula so that the glenoid cavity faces upwards and forwards during abduction of the arm. Although the trapezius produces the latter action, it is the serratus anterior that is the chief muscle involved in rotating the inferior angle of the scapula outwards and upwards. This muscle also holds the scapula to the thoracic wall during the excursions of this bone which is acted upon by a number of muscles attached to it. In the back of the arm, the radial nerve gives off most of its branches before it enters the groove for the radial nerve of the humerus, while only the lower lateral cutaneous nerve of arm and the posterior cutaneous nerve of forearm are given off as the nerve lies in the groove. Since the nerve is intimately related to the bone in this part of its course, fractures of the humerus about the middle of the shaft may damage the nerve. However, in the resultant paralysis, the triceps is unaffected. The axillary nerve may be injured in dislocation of the shoulder joint and in fractures of the surgical neck of the humerus with resultant loss of abduction of the arm.