322 ABRAM T. KERR 



THE ULNAR NERVE 



The ulnar nerve is usually described as arising from the medial 

 fasciculus of the plexus, and the medial fasciculus of the plexus 

 is usually stated to be formed by the junction of the eighth cer- 

 vical and first thoracic nerves and also at times to receive fibers 

 from the second thoracic nerve. 



In my series, the ulnar nerve is formed by the division of the 

 medial fasciculus of the plexus into ulnar nerve and medial head 

 of the median nerve, in 170 of the 175 plexuses studied, or in 

 97.14 per cent (figs. 1-8). 



In 5 instances, the ulnar nerve arises from a trunk formed by 

 the union of the medial fasciculus of the plexus with the whole 

 or a part of the lateral fasciculus. In 3 of these the trunk formed 

 by fusion of the medial and lateral fasciculi of the plexus almost 

 immediately breaks up into the ulnar, median, and musculo- 

 cutaneous nerves (fig. 27). In the other 2 cases, the lateral 

 head of the median joins the medial fasciculus of the plexus and 

 the trunk thus formed divides into ulnar and median nerves 

 (fig. 28). 



The lateral head of the ulnar nerve 



In some of the 170 plexuses in which the ulnar nerve arises 

 by division of the lateral fasciculus of the plexus, the nerve re- 

 ceives on its lateral side an additional branch. This has been 

 named the lateral head of the ulnar nerve, caput laterals nervus 

 ulnaris (figs. 1, 2, 3, 10). This lateral head of the ulnar nerve has 

 been figured and mentioned at intervals for a long time by 

 various authors. 



The size of the lateral head of the ulnar nerve is subject to 

 much variation, ranging from a minute thread to a good sized 

 branch, as large as the usual medial antibrachial cutaneous 

 nerve. It is often small and pierces or crosses the medial head 

 of the median nerve. There it is intimately bound up in the 

 connective tissue sheath of the latter so that its relation to the 

 ulnar nerve is easily overlooked. In separating the ulnar from 

 the median nerve in dissection, the connection of the lateral head 

 of the ulnar, if not particularly looked for, is often broken. 



