834 COMPARATIVE ANATOMY 



is only in the higher forms that it lies upon the dorsal. Several in- 

 genious explanations have been attempted but none is satisfactory. 

 Students should appreciate the fact that our elaborate nervous system 

 which controls every movement of the body, is one of the most highly 

 and elaborately protective systems we could possibly possess. 



The brain itself, the head-end of the nervous system, is enclosed in 

 a remarkable bony case, while the spinal cord (the continuation of the 

 brain, caudad), is encased within the slightly movable but nevertheless 

 well fitted vertebrae that make up the spinal column. The brain and 

 spinal, cord combined are called the central nervous system so as to dis- 

 tinguish it from the peripheral nervous system, which latter consists of 

 all those nerves arising from the brain and spinal cord. 



As the central nervous system is composed of an infolding of the 

 outer portion of the body, the ectoderm thus infolded into the central 

 portion of the neural tube, becomes the sensitive part of the central 

 nervous system. This sensitive surface lines the lumen of the neural 

 lube, and v/hile this condition remains in all higher forms, including 

 man, we shall see in our study of the brain that larger or smaller masses 

 of gray matter may migrate to various parts of the brain. 



It will be readily understood that a nervous system of this kind, 

 Avhich is well protected by a bony covering, has many advantages over 

 mere external tactile-sense spots, such as the earthworm possesses ; 

 still, to be of any value whatever, any inner sensory portion must retain 

 its connection with the outer portion of the body. It is such connections 

 which, when they have definite cells and processes that unite with the 

 central nervous system and are grouped together, become special sense 

 organs. Such nerve fibres, together with their cells, are known as 

 sensory nerves. Sensory nerves must therefore carry impulses from 

 Outer portions to innermost regions, or in other words, from the external 

 portions of the body to the central nervous system. 



The object of the nervous system is primarily to inform the animal 

 of the conditions, good and bad, in the environment, to correlate this in- 

 iformation, and to regulate the motion so that advantage may be had of 

 this knowledge. In those forms of animals which are segmented, that is, 

 in which metameres appear, especially when this metamerism is in the 

 mesothelium from which the myotomes develop into muscles, there are 

 usually one or more pairs of motor nerves going to these segments be- 

 cause each muscle must have its own nerve supply. The motor nerves 

 carry impulses from the central nervous system to the muscle or organ 

 in which they are placed. 



The close association of sensory and motor nerves in the trunk 

 region of vertebrates has not been satisfactorily explained. In 

 Amphioxus the two kinds of nerves are independent of each other 



