THE ARM AND ELBOW. 37 



The form of the back of the arm is determined by the triceps muscle, the three 

 iieads of which, together with the large tendon of insertion, are to be recognized 

 when the muscle is called into play. The inner head is the least distinct ; the 

 outer head forms a large prominence immediately below the hinder border of the 

 deltoid ; the long head can be seen issuing from between the teres major and minor 

 muscles, and descending along the middle of the back of the arm ; while the tendon 

 is represented by a depressed area, leading down to the olecranon process of the 

 ulna. The musculo-spiral nerve begins to incline backwards immediately below the 

 posterior fold of the axilla, and crosses the back of the humerus obliquely from 

 within outwards in its middle third, being covered by the long and outer heads of 

 the triceps muscle, and accompanied by. the superior profunda vessels. At, or a 

 little above, the junction of the middle and lower thirds of the arm, the nerve 

 perforates the external intermuscular septum, and it then descends in front of the 

 outer supracondylar ridge, and under cover of the supinator longus muscle, to the 

 level of the external condyle, where it divides into the radial and posterior interos- 

 seous nerves. The former takes a straight course downwards to join the artery of 

 the same name below the elbow ; but the posterior interosseous is directed back- 

 wards across the outer side of the radius in its upper fourth, to gain the back of 

 the forearm. 



THE ELBOW. 



At the elbow, the internal and external condyles come to the surface, and also 

 the olecranon process of the ulna. The internal condyle, which, it will be 

 remembered, is directed more backwards than inwards, is very prominent, and forms 

 one of the most important bony landmarks of the limb. The external condyle, 

 together with the common tendon of the extensor muscles of the forearm, gives rise, 

 when the joint is extended, to a well-marked depression at the outer and back part 

 of the elbow, between the supinator longus and extensor carpi radial is longior 

 muscles externally, and the anconeus internally. In this hollow, when the muscles 

 are relaxed, the head of the radius may be felt below the external condyle and the 

 capitellum. If the elbow be semi-flexed, the condyle is slightly prominent ; and in 

 extreme flexion, the outer part of the triceps muscle is stretched over the capitellum 

 of the humerus, which forms a rounded eminence to the outer side of the point of 

 the elbow (olecranon), while the condyle itself is no longer visible. The olecranon 

 is subcutaneous at its posterior surface, its upper end being entirely covered by the 

 insertion of the triceps ; its appearance necessarily varies with the position of the 

 joint, as does also the distance between the process and the shoulder. A bursa is 

 interposed between the bone and the skin. 



At the bend of the elbow, the subcutaneous veins are more or less distinctly 

 visible, according to the quantity of subcutaneous fat : the median vein bifurcat- 

 ing into the median-basilic and median-cephalic, which join respectively the 

 ulnar and radial veins to form the basilic and cephalic. The median-basilic and 

 median-cephalic veins, diverging from each other, pass upwards on either side of 

 the biceps tendon, which is seen, when the elbow is bent, descending from the lower 

 end of the muscular belly into the interval between the two masses of forearm 

 muscles. The sharp upper edge of the bicipital fascia may also be felt, and, when 

 the muscle is forcibly contracted, seen, as it passes downwards and inwards between 

 the median-basilic vein and the lower part of the brachial artery. The pulsation of 

 the latter vessel may be felt, and often seen, as it passes obliquely downwards and 

 outwards to a point a little below the middle of the bend of the elbow. 



