SPINAL NERVES 233 



branch supplies the viscera and is in connection with the 

 sympathetic. 



1. SPINAL NERVES. 



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

 nerves or nerve-roots lies in the same transverse plane : an excep- 

 tion to this is seen, however, in Cyclostomes, Elasmobranchs, and 

 Dipnoans, in which each ventral pair alternates with a dorsal 

 pair. 1 In Ganoids also, lateral displacements 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, or between the arches ; but from Amphi- 

 bians onwards the nerves always make their exit on either side 

 between the arches, through the iritervertebral foramina. 



In their primitive undifferentiated condition the spinal nerves 

 have a strictly metameric arrangement, and are equally developed 

 all along the body. In the region of the developing extremities, 

 the nerves bifurcate and extend into the limb, each forming a 

 dorsal extensor and a ventral flexor. This primary condition 

 becomes complicated by the fact that the extensions of the 

 myotomes into the limbs do not all become differentiated simply 

 into a dorsal and a ventral portion, and thus it becomes impossible 

 to draw a hard and fast line between the dorsal and ventral 

 branches of the nerves of the extremities. As a rule, a number of 

 the ventral branches of the spinal nerves in the region of the 

 appendages become connected together to form plc.mses which, 

 according to their position, are spoken of as cervical, brachial, 

 lumbar, and sacred (Figs. 145 and 147). 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 development of the 

 latter. A further important factor in the formation of the limb- 

 plexuses can be traced to the ontogenetic and phylogenetic shifting 

 of the extremities along the trunk, whereby they assimilate nerves 

 from other segments and lose some of those which primarily 

 belonged to them. 



In contrast to Fishes, the great variation in the comparatively 

 slightly developed 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 cervico-brackial and lumbo- 



1 In Amphioxus, the dorsal and ventral nerves alternate right and left, so 

 that a right dorsal and a left ventral nerve are in the same transverse plane and 

 vice versa, and the fibres composing the ventral nerves are not surrounded by a 

 common sheath. The dorsal nerves of Amphioxus and Petromyzon, like many 

 of the cerebral nerves of all Craniata, are throughout of a mixed nature, and this 

 was possibly the case in the Protovertebrata. 



