SPINAL NERVES 179 



roots divides up again into a dorsal, a ventral, and a visceral 

 branch. The first of these goes to the muscles and skin of the 

 back, the second supplies the lateral and ventral portions of the 

 body-wall, while the intestinal branch comes into connection with 

 the sympathetic (p. 188). 



1. SPINAL NERVES. 



As a general rule, each corresponding pair of dorsal and ventral 

 roots lies in the same transverse plane : an exception to this is 

 seen, however, in Amphioxus, 1 Cyclostomes, Elasmobranchs, and 

 Dipnoans, in which the mesoblastic somites of the right and left 

 side are arranged alternately, and thus the points of exit of the 

 nerve-roots also alternate right and left, or each ventral pair 

 alternates with a dorsal pair. In Ganoids also, lateral displace- 

 ments of the nerve-roots are met with. 



In Fishes the greatest variations are seen as regards the mode 

 of exit of the nerves (which pass through the intercalary pieces 

 of the vertebral column, through the arches, or between them) ; 

 but from the Amphibia onwards the nerves always make their 

 exit on each side between the arches, through the intervertebral 

 foramina. 



In their primitive undifferentiated condition the spinal nerves 

 have a strictly metameric arrangement, and are equally developed 

 in all regions of the body. As already pointed out in the section on 

 the spinal cord, this condition becomes modified by the development 

 of the appendages, so that a number of spinal nerves unite together 

 to form plexuses, which, according to their position, are spoken of 

 as cervical, brachial, lumbar, and sacral (Fig. 121). The number 

 of nerves composing these corresponds to the number of body- 

 segments taking part in the formation of the appendages, and 

 their relative size is usually directly proportional to the develop- 

 ment of the latter. 



In contrast to Fishes, the great variation in the plexuses of 

 which renders it impossible to reduce them to a common plan, we 

 find from the Amphibia onwards a typical grouping of the branches 

 of the brachial plexus, from which numerous nerves arise supplying 

 the shoulder and fore-limb dorsally and ventrally (e.g., thoracic, 

 subscapular, axillary, radial, musculo-cutancous, and ulnar}. The 

 lumbo-sacral plexus shows in general, and more particularly in 

 Mammals, much greater variations than does the brachial plexus. 

 The nerves arising from it are also arranged in a dorsal and a 

 ventral series, the larger ones being spoken of as the obturator. 



1 In Amphioxus both the dorsal and ventral nerves innervate muscles, and 

 it appears that in many of the Craniata also the dorsal roots are not purely 

 sensory. 



N 2 



