IX. ] THE NER VO US S YSTEM. 



401 



besides this each gives off a minute filament (or a pair) to 

 what is called the sympathetic system. 



Of these two series of conspicuous branches, it is the 

 ventral one which constitutes the nerves of the limbs. In 

 the part of the body intervening between the pectoral and 

 pelvic limbs, the ventral branches pass, in the thoracic 

 region, between the ribs, as intercostal nerves ; and in the 

 abdominal region they run along between the internal oblique 

 and the transversalis muscles. 



It may be, by rare exception (as in the Ganoid Fishes), that 

 the dorsal and ventral nerves, instead of being the bifurcating 

 branches from a common root, are distinct nerves, each 

 arising by its own root. 



28. The NERVES OF THE ARM of man result from the inter- 

 mixture, in what is called the brachial plexus, of the ventral 

 branches of the four lowest cervical and the first dorsal 

 nerves. 



Issuing from the plexus, the most noteworthy nerves are 

 the ulnar and median, which supply the pronator muscles 

 and the flexors of the digits, and the musciilo-spiral nerve, 

 which supplies the supinator muscles and the extensors of 

 the digits. The last-mentioned nerve comes round the back 

 of the humerus to pass along towards the radial side of the 

 front of the fore-arm. The median nerve, passing down the 

 inner side of the upper arm, crosses to the front of the elbow 

 and descends along the middle of the fore-arm. The ulnar 

 nerve passes down between the olecranon and inner condyle 

 of the humerus. 



The position and number of those spinal nerves which 

 supply the limbs may be very different from what we find in 

 man. Thus, in Birds (which often have so many cervical 

 vertebrae) the limb nerves are more remote from the head, 

 while in Batrachians they are closer to it, and the brachial 

 plexus may be formed but by two. 



Nevertheless a general resemblance exists with regard to 

 the nerves of the arm in all Vertebrates above Fishes, ex- 

 cepting, of course, those animals in which the pectoral limbs 

 are absent or rudimentary. 



In Fishes, however, we find very different conditions, 

 (i) We may have, as in many osseous Fishes, but the first two 

 pairs of spinal nerves for the supply of the pectoral fin ; or 

 (2), as in the Rays, we may have an enormous number of 

 spinal nerves (much more than half those of the entire body), 

 which, though collected and connected in different bundles, 

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