162 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



to the parts to which the fibres are distributed, causing these to act- 

 muscles to contract, glands to secrete, etc. Hence the ventral roots are 

 called motor roots. Their fibres are without sensory functions, while 

 sensory fibres are equally unable to cause action in any peripheral 

 part (Bell's law). 



After a longer or shorter course, each spinal nerve, formed by the 

 union of dorsal and ventral roots, divides into three branches, each of 

 which receives both sensory and motor fibres. These are known as 



FIG. 165. A, diagram of collector nerve; B, of a nerve plexus, after Braus; C, branchial 

 plexus of Sa/amadra maculata, after Flirb ringer. 



the ramus dorsalis, ramus ventralis and ramus visceralis or in- 

 testinalis. The first goes to the skin and muscles of the dorsal region; 

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

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

 the mesentery, where it connects with the sympathetic nervous system 

 to be described below (fig. 166). 



Recent physiological and histological analysis shows the existence 

 of two groups of nervous elements in both sensory and motor nerves. 

 There are somatic sensory and motor fibres, distributed to the skin 

 and most of the external sense organs and to the voluntary muscles, and 



