648 NER\ES OF THE UPPER LIMB. 



and this small communicating filament represents in such cases the nerve of 

 Wrisberg. 



MUSCULO-CUTANEOUS NERVE. 



The musculo-cutaneous or external cutaneous nerve (perforans Casserii) in 

 deeply placed between the muscles as far as the elbow, and below that point 

 is immediately under the integument. Arising from the brachial plexus 

 opposite the small pectoral muscle, it perforates the coraco-brachialis muscle, 

 and, passing obliquely across the arm between the biceps and brachialis 

 anticus muscles, reaches the outer side of the biceps a little above the 

 elbow. Here it perforates the fascia, and nearly opposite the elbow-joint it 

 passes behind the median-cephalic vein, and, inclining outwards, divides 

 into two branches which supply the integument on the outer side of the 

 forearm, one on the anterior, the other on the posterior aspect. 



A. Branches in the arm : 



a. A branch to the coraco-brachialis and short head of the biceps is given off before 

 the nerve pierces the former muscle ; and other filaments are furnished to the coraco- 

 brachialis, while the nerve lies among its fibres. 



b. Branches to the biceps and brachialis anticus are given off while the nerve is 

 between those muscles. 



c. Small filaments are given to the humerus and elbow-joint. 



B. Branches in the forearm : 



a. The anterior branch descends near the radial border of the forearm. It is 

 placed in front of the radial artery near the wrist, and distributes some filaments 

 over the ball of the thumb. Piercing the fascia, it accompanies the artery to the 

 back part of the carpus. This part of the nerve is connected at the wrist with a 

 branch of the radial nerve. 



b. The posterior branch is directed outwards to the back of the forearm, and 

 ramifies in the integument of the lower third, extending as far as the wrist. It com- 

 municates with a branch of the radial nerve, and with the external cutaneous branch 

 of the musculo-spiral nerve. 



Summary. The musculo-cutaneous nerve supplies the coraco-brachialis, 

 biceps, and brachialis anticus muscles, and the integument on the outer 

 side of the forearm. Communications are established between it and the 

 radial and the external cutaneous branch of the musculo-spiral. 



Varieties. In some cases it does not perforate the coraco-brachialis 

 muscle. It is frequently found to communicate by a cross branch with or to 

 be an offset of the median nerve ; and in the latter case, the coraco-brachialis 

 muscle receives a separate branch from the brachial ploxus, which may be 

 explained thus, that the main part of the musculo-cutaneous nerve, instead 

 of piercing the coraco-brachialis muscle, remains adherent to the outer root 

 and trunk of the median. 



ULNAR NERVE. 



The ulnar nerve, the largest branch of the inner cord of the brachial 

 plexus, descends on the inner side of the artery of the limb as far as the 

 middle of the arm, then turns backwards through the internal intermuscular 

 septum with the inferior profunda artery, to reach the interval between the 

 olecranon and the inner condyle of the humerus. From the axilla to this 

 place it is covered only by the fascia, and it may be felt through the integu- 

 ment a little above the elbow. It here passes between the two heads of 

 the flexor carpi ulnaris, and it remains concealed by that muscle as far as the 

 middle of the forearm ; it thence extends in a straight course along the' 

 outer margin of the muscle, between it and the uluar artery, to the outer 



