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 
iti ll 
S 
| Pee 
Fic. 165.—A, diagram of collector nerve; B, of a nerve plexus, after Braus; C, branchial 
plexus of Salamandra maculata, after Fiirbringer. 
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 
