PERIPHERAL NERVOUS SYSTEM 1 75 



second to those of the sides and ventral parts of the body; while the 

 visceral branch descends to the roof of the coelom, near the insertion 

 of the mesentery, where it connects with the sympathetic nervous 

 system to be described below (fig. 184). 



Recent physiological and histological analysis shows the exist- 

 ence of two groups of nervous elements in both sensory and motor 

 nerves (fig. 184). There are so matic se nsory and somatic motor 

 fibres, distributed to the skin and most of the external sense -ol-gans 

 and to the voluntary muscles, and there are also visceral fibres of 

 both kinds, supplying the viscera (alimentary canal, excretory and 

 reproductive organs) and the circulatory system. The dorsal and 

 ventral rami contain mostly somatic fibres with a few of the visceral 

 type, while the visceral rami are composed of visceral fibres alone. 

 The farther subdivision of these nerves will be considered later. 



To the statement that the dorsal roots are purely sensory the exception must 

 be made that in the lower vertebrates some of the visceral motor fibres, arising 

 in the neighborhood of the lateral column, pass out from the cord through the 

 dorsal root (fig. 184, A). In the mammals they are said to leave by the ventral 

 roots like all other motor fibres (fig. 184, B). 



Spinal nerves are grouped in description according to the region of the 

 vertebral column in which they leave the spinal cord. They are numbered 

 within each region from the vertebra which succeeds them, except that the first 

 nerve behind the skull is the first cervical, with the result that in the cervical 

 region there is one more nerve than the number of vertebrae. 



In the regions of the appendages the spinal nerves usually form 

 networks or plepises, branches of a varying number of ventral rami 

 interlacing in a complicated manner before entering the appendage 

 (fig. 185 B, C). In the lower vertebrates there are two plexuses on 

 either side, a cervico-brachial for the fore limb and a lumbo-sacral for 

 the posterior appendage, these dividing in the higher groups into 

 cervical, brachial, lumbar and sacral plexuses, these receiving their 

 names from the neck, fore limb, and lumbar and sacral regions 

 respectively. 



The number of nerves which enter a plexus varies between wide 

 limits, the largest number (about twenty-five) being found in the 

 skates, in correlation with the great development of the pectoral fin. 

 It is of interest that in the snakes and limbless lizards there is a 

 plexus, an indication that these have descended from limbed an- 

 cestors, though no other traces of limbs occur in development. 

 From the plexus arise the nerves which enter the limb, there being 

 dorsal and ventral branches for the two sides of the limb. In the 



