CHAPTER XVII 



NERVOUS SYSTEM 



The nervous system is composed of cells specialized for the 

 transmission of impulses, and acts as the coordinating mecha- 

 nism of the body. Stimuli from one region are transmitted to other 

 regions, and through the function of the nerves the body is made 

 to respond as a unified whole. So closely are the muscles cor- 

 related with the nerves that the two are physiologically grouped 

 as the neuro-muscular system. Even the involuntary bodily re- 

 actions are regulated by the nervous system. 



A. Development 



Nerve tissues are derived from the ectoderm along the dorsal 

 portion of the embryo. The nervous sj^stem begins as a neural 

 groove anterior to the blastopore (page 23). The lateral ridges 

 of the groove grow medially until a tube is formed. The develop- 

 ment progresses more rapidly at the anterior end, with the result 

 that the brain becomes specialized into regions before the posterior 

 end of the groove has closed to form a tube. As the neural tube 



Neural groove 



Neural crest Somatopleure 

 'Ectoderm 



Epimere ' Endokerm 

 Notochord Splanchnopleure 



Fig. 152. Cross Section of Chick Embryo. 

 271 



