PLATE III. 

 THE BRACHIAL PLEXUS. 



This plate illustrates a deep dissection of the axilla and the lower part of the 

 posterior triangle of the neck. The middle third of the clavicle has been removed 

 and the subclavius muscle divided ; the weight of the limb has carried it backwards, 

 exposing and tensing the cords of the brachial plexus. 



The posterior scapular artery arises in this instance from the third part of the 

 subclavian artery : this happens in about 40 per cent, of the bodies examined, and thus 

 may be given as one of its normal modes of origin ; otherwise, it is usually said to arise 

 from the transversalis colli branch of the thyroid axis. 



The brachial plexus in the plate differs a little from that usually taught, for 

 the anterior branch of the seventh cervical sends a twig to the inner cord of the 

 plexus ; this can be traced almost entirely into the median nerve, of which it is really 

 an aberrant root ; while the remainder, instead of joining the outer cord, received a 

 communicating branch from it, and ran on by itself to join the median. The external 

 anterior thoracic arose from the sixth and seventh cervical by two distinct roots, and 

 not from the outer cord as is usually given. 



Note on tlie Limb Plexuses, A few words may be necessary to explain the 

 terms anterior and posterior divisions used in the description of the limb plexuses. 

 Each spinal nerve as it exits from the canal divides into two primary divisions, 

 a dorsal or posterior primary division which supplies the tissues of the back, and 

 an anterior or ventral division which supplies those in the front. It is the anterior 

 (inferior of quadrupeds) primary division that alone enters into the limb plexuses. 



To go back to the development of the limb, in its primitive state as it buds out from 

 the trunk it has two surfaces, a ventral and dorsal, each of which has its own nerve 

 supply, and thus the anterior primary division splits at once into two trunks, an anterior 

 or ventral, and a posterior or dorsal, for the supply of the respective surfaces. This 

 simple arrangement is well seen in the Amphioxus lanceolatus. In the embryo rat the 

 same thing occurs, all the nerves dividing in this manner, but in the adult rat this 

 arrangement is masked, as development proceeds the embryonic nerves becoming 

 differentiated and complicated in their arrangement pari passu with the development 

 of the muscular system, and the changes to the adult condition. 



In all mammalian limb plexuses it will be found that : 



1. The inferior (anterior) primary divisions of the spinal nerves entering the 

 plexus divide into anterior and posterior trunks. 



2. That dorsal trunks always unite with dorsal, and ventral with ventral, to form 

 the nerves of distribution. 



8. The essential constitution of a nerve of distribution never varies, i.e. a nerve 

 coming from a dorsal trunk in one animal never comes from a ventral trunk in another. 1 



Birmingham considers that the lesser internal cutaneous (N. of Wrisberg) is the 

 posterior or dorsal division of the first dorsal nerve, because when the nerve is absent 

 its place is taken by a lateral cutaneous from first dorsal, the portion of the first dorsal 

 which passes up over the first rib to join the brachial plexus then passing at once into 

 the anterior (ventral) division of the nerves ; also the first intercostal does not fully 

 represent the ventral element of first dorsal, because it usually gives no anterior 

 cutaneous branch. 2 



1 Dr. Patterson, Journal of Anatomy and Physiology, vol. iii. p. 612. 

 * Journal of Anatomy and Physiology, vol. xxx. p. 03. 



