THE BRACHIAL PLEXUS. 1303 



9. The Internal Anterior Thoracic Nerve. 



The internal anterior thoracic nerve (n. thoracalis anterior medialis) (Fig. 

 1093) arises from the inner cord and consists of fibres derived from the eighth 

 cervical and first thoracic nerves. It passes forward between the axillary artery and 

 vein and, after giving of? a branch which forms a loop with a similar branch from the 

 external anterior thoracic, pierces the pectoralis minor, in which some of its fibres 

 terminate. The remainder enter the deep surface of the pectoralis major to supply 

 the lower part of the sternal portion of that muscle. 



Variations. — The fibres which supply the pectorahs major may wind around the lower 

 border of the pectoralis minor. Filaments from both of the anterior thoracic nerves may supply 

 the integument of the axillary and mammary regions. 



10. The Lesser Internal Cutaneous Nerve. 



The lesser internal cutaneous nerve (n. cutaneus brachii medialis) (Fig. 1093), 

 also called the nerve of Wrisberg, can be traced to the first thoracic nerve. It 

 arises from the inner cord usually in common with the internal cutaneous. After 

 leaving its point of origin, it descends in the arm along the inner side of the axillary 

 and basilic veins, pierces the deep fascia about the middle of the arm and supplies 

 the integument of the inner aspect of the upper extremity as far down as the elbow. 

 At a variable point it forms a loop with the intercosto-humeral nerve. 



Variations. — The lesser internal cutaneous nerve may be absent. It may receive fibres 

 from the eighth cervical or the second thoracic nerve. There may be present a communication 

 between the lesser internal cutaneous nerve and the lateral cutaneous branch of the third tho- 

 racic. The inosculation with the intercosto-humeral may be either simple or plexiform and 

 either" nerve may be deficient, the other usually recompensing for the deficiency. 



II. The Internal Cutaneous Nerve. 



The internal cutaneous nerve (n. cutaneus antebrachii medialis) (Fig. 1094) 

 comprises fibres from the eighth cervical and first thoracic nerves. It has its origin 

 from the inner cord of the plexus usually as a common trunk with the lesser internal 

 cutaneous nerve. After distributing some small filaments to the integument of the 

 upper arm below the axilla, it runs down the arm between the brachial artery and 

 the basilic vein and at about the middle of the upper arm breaks up into its terminal 

 branches, {a) the ati/erior a.nd {d) X\\e posterior. 



a. The anterior branch (r. volaris) passes over, sometimes under, the median-basilic 

 vein and supplies the skin of the ulnar half of the forearm as far down as the wrist (Fig. 1104). 

 It inosculates with the superficial branch of the ulnar nerve. 



b. The posterior branch (r. ulnaris) turns obliquely around the inner side of the upper 

 part of the forearm and supplies the integument as far around as the ulna down to the lower 

 third or fourth of the forearm. It unites above the elbow with the lesser internal cutaneous 

 nerve and in the forearm with the anterior branch of the internal cutaneous and sometimes with 

 the dorsal ramus of the ulnar. 



12. The Ulnar Nerve. 



The ulnar nerve (n. ulnaris) (Fig. 1092) is the largest branch of the inner 

 cord. Its fibres can be traced to the eighth cervical and first thoracic nerves and 

 sometimes, by a root from the outer cord, to the seventh cervical. Arising from 

 the inner cord between the axillary artery and vein and posterior to the internal 

 cutaneous nerve it pursues a downward course in front of the triceps and to the 

 inner side of the axillary and brachial arteries. Reaching the middle of the arm it 

 follows an inward and backward direction, in which it is accompanied by the inferior 

 profunda artery, and passing either over the inner margin of or through the internal 

 intermuscular septum and in front of the inner head of the triceps, attains the interval 

 between the internal condyle of the humerus and the olecranon (Fig. 1098). It 

 becomes an occupant of the forearm by passing between the heads of the flexor carpi 

 ulnaris muscle, a situation the nerve shares with the inferior profunda and posterior 



