14 THE ANATOMY OP THE HOESE. 



medium head of the triceps and the anconeus, and furnishes a few 

 cutaneous branches which perforate the caput medium, or emerge at its 

 lower part, to be distributed to the skin on the outer side of the fore- 

 arm, below the elbow. The termination of the nerve will afterwards be 

 followed in the fore-arm, where it supplies the extensor muscles and the 

 flexor metacarpi externus. 



The Ui,NAR Nerve (Plates 5 and 6) derives its fibres from the dorsal 

 roots of the brachial plexus. At first it lies close behind the main 

 vessels ; but as it passes downwards it recedes from them, and, passing 

 under cover of the scapulo-ulnaris, it reaches the space between the 

 olecranon and the inner condyle. Thence it descends to the back of 

 the fore-arm, where it will subsequently be dissected. At present it is 

 seen to give off only one branch, which disappears within the superficial 

 pectoral muscle, and afterwards becomes distributed to the skin of the 

 fore-arm (Plate 5). 



The Median Nerve (Plates 5 and 6) is formed by the union of two 

 roots. The anterior of these comes from the 6th, 7th, and 8th cervical, 

 while the posterior is derived from the 8th cervical and the 1st dorsal. 

 These roots give off some pectoral twigs, and then unite by forming a 

 loop in which the axillary artery rests. The nerve then descends in 

 front of the axillary artery and its brachial continuation, and will after- 

 wards be seen to accompany the posterior radial artery. The following 

 branches of the nerve may be found at present : — 



The Nerve to the Biceps and C'oraco-humeralis comes off close below 

 the union of the two roots of the median, or from the anterior root 

 above the point of iinion. It passes between the upper and lower in- 

 sertions of the coraco-humeralis, supplying that muscle and terminating 

 in the biceps. 



Muscido-cutaneous branch. — This is given off from the median about 

 the middle of the humerus ; and, passing underneath the biceps, it 

 divides into a muscular branch for the brachialis anticus, and a 

 cutaneous branch for the front of the fore-arm. 



Directions. — The muscles of this region should now be examined in 

 the order of their description. 



The Latissimus Dorsi (Plates 5 and 6). The insertion of this muscle 

 into the inner tubercle of the humerus is here seen. About an inch or 

 two from its termination the tendon gets a twist which alters the 

 direction of its surfaces, and brings it to be inserted in front of the 

 termination of the teres major on the same tubercle. 



Action. — The muscle is a flexor and an inward-rotator of the shoulder- 

 joint. 



The Teres Major (Plate 5). It arises from the dorsal angle of the 

 scapula, and from an aponeurosis between it and the subscapularis. It 

 is inserted into the internal tubercle of the humerus, its terminal tendon 



