716 THE NEEVOUS SYSTEM. 



The nerve supplies the following branches : 



1. Muscular branches to the muscles of the second intercostal space. 



2. Cutaneous branches, (a) Anterior terminal branches to the skin over the 

 second intercostal space (Fig. 623). (6) A large lateral cutaneous branch, the intercosto- 

 brachial (O.T. intercosto-humeral) nerve (Fig. 614, p. 701). This nerve pierces the 

 intercostal and serratus anterior muscles, and, crossing the axilla, extends to the arm. 

 It pierces the deep fascia just beyond the posterior fold of the axilla, and can be traced 

 as far as the interval between the medial epicondyle of the humerus and the olecranon. 

 It supplies an area of skin stretching across the axilla and along the posterior surface of 

 the arm on the medial side as far as the elbow (Fig. 617, p. 706). It may supply the 

 axillary arches, when present. 



The intercosto-brachial nerve varies in size. It may pierce the first intercostal space, and it 

 is often divisible into anterior and posterior branches, like the lateral branch of an ordinary 

 intercostal nerve. 



Communications. (1) The intercosto-brachial nerve communicates with two adjacent 

 nerves. Either before or after piercing the fascia of the axilla it is joined by the medial 

 cutaneous nerve of the arm. It also communicates with the posterior part of the lateral 

 branch of the third intercostal nerve by means of the branches distributed to the floor and 

 boundaries of the axilla. (2) Besides the branches referred to, the second thoracic nerve 

 in many cases transmits a nerve to the brachial plexus, which becomes incorporated with 

 the first thoracic nerve after passing over the neck of the second rib. This branch is 

 inconstant. As already mentioned, it may join only the intercostal part of the first 

 thoracic nerve, it may join the brachial plexus only, or it may send branches to both 

 parts of the first thoracic nerve. (3) Besides the communications effected by branches 

 of the second thoracic nerve in its course, it also receives a gray ramus communicans 

 from the second thoracic ganglion of the sympathetic trunk in the thorax. It also sends 

 to the sympathetic a white ramus communicans, probably the first, though this is not 

 known with certainty. 



The anterior ramus of the third thoracic nerve differs from a typical thoracic 

 nerve only in one respect. Its lateral branch divides in the usual way into 

 anterior and posterior parts, of which the latter is carried to the arm and supplies 

 an area of skin on the medial side near the root of the limb. It effects a junction 

 with the intercosto-brachial nerve (Fig. 614, p. 701). 



The anterior rami of the fourth, fifth, and sixth thoracic nerves have a course 

 and distribution which is simple and typical. Except for the peculiarities above 

 mentioned, the second and third thoracic nerves have a similar distribution. 



The nerves lie on the posterior wall of the thorax, in the costal groove of the 

 corresponding rib. They extend forwards between the intercostal muscles as far 

 as the middle of the chest wall, lying at a lower level than the intercostal vessels. 

 At the side of the chest each nerve passes obliquely through the internal intercostal 

 muscle, and comes to lie upon the pleura, transversus thoracis muscle, and 

 internal mammary artery. Thereafter, piercing the fibres of the internal inter- 

 costal muscle, the aponeurosis of the external muscle, and the pectoralis major, 

 each nerve ends by supplying the skin of the front of the chest, over an area 

 corresponding to the medial or anterior part of the intercostal space to which 

 it belongs. 



Branches. Each intercostal nerve supplies, in addition to the anterior terminal 

 cutaneous branches, muscular branches to the intercostal muscles and a lateral 

 cutaneous ramus, which, piercing the intercostal and serratus anterior muscles, 

 divides into anterior and posterior branches for the innervation of the skin over 

 the side of the chest. Each area of skin thus innervated is continuous anteriorly 

 with the area innervated by the anterior rami of the same nerves, and posteriorly 

 with the areas supplied by their posterior rami. 



The upper six intercostal nerves supply the muscles of the first six intercostal 

 spaces and the transversus thoracis (3, 4, 5, 6). The second, third, fourth, fifth,! 

 and sixth nerves supply the skin of the front of the chest : the second, opposite 

 the sternal synchondrosis ; the sixth, opposite the base of the xiphoid process.: 

 Their lateral branches supply branches to the intercostal muscles and the skin of 



