42 THE SKATE 



II. THE NERVOUS SYSTEM. 



The function of the nervous system is that of conduction, 

 coordination and correlation of stimuli. 



The nervous system may be divided into three parts: 

 (1) the central nervous system, composed of the brain 

 within the skull and the spinal cord within the neural canal 

 of the vertebrae; (2) the peripheral nervous system, com- 

 posed of the cranial nerves from the brain and the spinal 

 nerves from the spinal cord; (3) the sympathetic system 

 which controls and regulates the involuntary activities and 

 the organs involved, such as the heart, digestive tract, 

 respiratory and reproductive systems, and the smooth mus- 

 culature in general. It is connected with outgrowths of 

 the peripheral nervous system. 



The brain and spinal cord are composed of nerve-cell 

 bodies (gray matter) and nerve-cell processes (white mat- 

 ter). The cranial and spinal nerves are made up of proc- 

 esses only. 



1. The Brain. 



The central nervous system is developed from the tube 

 formed originally by an infolding of the ectodermal medul- 

 lary plate of the embryo. During the development of the 

 brain there are at first three vesicles, which have been 

 named the forebrain or prosencephalon, the midbrain or 

 mesencephalon, and the hindbrain or rhombencephalon. 

 The first and third subsequently divide into two. Each 

 of the five divisions is further changed by evaginations, 

 foldings and thickenings. The adult brain has, however, 

 the ^Ye main divisions : the telencephalon ; the diencepha-j 

 Ion or thalamencephalon ; the midbrain or mesencephalon ; 

 the metencephalon or cerebellum; and the mylencephalon 

 or medulla oblongata. 



Remove the skin from the top of the head back to a 



