754 THE NERVES. 



DIAPHRAGMATIC (OE PHEENIc) NEEVB. 



The diaphragmatic nerve (the internal respirator^/ nerve of Bell) is formed 

 by two principal branches, and a small accessory ramuscule whose presence 

 is not constant. The latter comes from the fifth cervical pair ; the two 

 others proceed, one from the next pair, the other from the brachial plexus. 

 The branch from the sixth pair pierces the inferior scalenus muscle from 

 within to without, gives off a filament to the brachial plexus, and descends 

 obliquely backwards to the surface of the muscle it passes through, to unite, 

 at the entrance to the chest, with the branch of that plexus. This latter 

 branch, generally shorter and thicker, comes exclusively from the seventh 

 cervical pair. . 



The trimk of the diaphragmatic nerve, formed in this manner, after 

 receiving the brancli of the fifth pair — when it exists, passes within the 

 axillary artery, along with the pneumogastric nerve, and often at this point, 

 though not always, obtains a filament from the great sympathetic. It then 

 gains the side of the base of the heart, passing beneath the pleura, and 

 finally attains the phrenic centre, after a course of at least eight inches 

 between the two laminas of the posterior mediastinum — the left nerve being 

 in the proper mediastinum, while that of the right side lies in the serous 

 septum intended for the posterior vena cava. 



Even before its arrival at the aponeurotic centre, this nerve divides into 

 several branches, whose ramifications pass to the sides of the pillars of the 

 muscle. 



BRACHIAL PLEXUS. 



This plexus comprises an enormous fasciculus of nerves, situated between 

 the thoracic parietes and the inner face of the anterior member, formed by 

 the inferior branches of the sixth, seventh and eighth cervical, and the two 

 first dorsal pairs, and principally destined to the muscles and integuments 

 of that limb. 



Mode of constitution. — The sixth cervical pair only assists in the forma- 

 tion of this plexus by the slender filament from its diaphragmatic branch ; 

 but the next two are entirely devoted to it, as well as the first dorsal, with 

 the exception of a very thin ramuscule, which constitutes the first intercostal 

 nerve. The root furnished by the second dorsal pair only represents a very 

 small part of its inferior branch, the other portion forming a somewhat 

 voluminous intercostal nerve. 



The various branches converge towards each other, and gain the interstice 

 between the two portions of the scalenus muscle (if we consider it as one), 

 where they unite, and become confounded into a single fasciculus by sending 

 filaments and ramuscules to each other , this fasciculus soon separates into 

 a certain number of divisions, whose disposition will be referred to presently. 

 It will be remarked that the intercrossing of the branches composing the 

 brachial plexus does not occur in a confused and irregular fashion, and 

 if the reticulation of the ramuscules passing from one to another does not 

 take place in a constant manner, it is, at any rate, far from being inextricable. 

 It is easy to follow the filaments from any pair of nerves for a certain 

 distance in the divisions given off by the brachial plexus, especially after 

 maceration in dilute nitric acid. This originating fasciculus of the brachial 

 plexus is very wide and short It is at first comprised between the superior 

 portion of the scalenus (or superior scalenus) and the longus colli. In 

 passing between the two portions of that muscle, it bends round the first rib 



