COORDINATION IN ANIMALS 191 



region, with its brain, battery of sense organs, and 

 skull, from a trunk region with its spinal cord, vertebral 

 column, paired limbs, etc. This naturally involves a cor- 

 responding shifting and modification of the primitive con- 

 dition of the paired nerves; especially since the innervation 

 of a group of cells in normal development is apparently 

 rarely changed a nerve following the part which it origi- 

 nally supplied through many of the transformations and 

 even migrations of the latter. 



If this point of view is accepted, the cranial and spinal 

 nerves are, historically considered, similar structures. But 

 the former, synchronously with the changes in the head 

 region, have departed somewhat widely from their ancestral 

 condition and have even been augmented by nerves of diverse 

 origin. The spinal nerves, on the other hand, continue to 

 issue from the cord at about equal intervals and in metameric 

 arrangement as indicated by muscle segments and skeletal 

 structures, although those of certain regions unite in the body 

 cavity to form PLEXUSES for the adequate innervation of the 

 appendages. 



From the standpoint of function the nerves are of two 

 classes, SENSORY and MOTOR. The former are distributed 

 mainly to the skin and sense organs of the head, and are the 

 paths over which excitations (NERVOUS IMPULSES) due to 

 external stimuli are conducted to the cord and brain. The 

 motor nerves, on the other hand, are the media for distribut- 

 ing impulses from the central organ to muscle cells, gland 

 cells, etc., and thus induce the response of the organism. 



In discussing nerves, it must be kept in mind that a nerve 

 is actually a bundle of nerve fibers; the fibers themselves in 

 turn being prolongations of nerve cells, the cell bodies of 

 which are usually situated in groups or GANGLIA. Moreover, 

 nerve impulses are not transmitted by nerves as a whole, but 



