NERVOUS SYSTEM. 137 



THE NERVOUS SYSTEM. 



Nervous and sensory structures are closely related to each other, 



and their distinction in the higher animals is the result of differentiation 



among cells which were originally both nervous and sensory in character, 



and it is in this broader sense that the term nervous structures is used 



in these introductory paragraphs. 



The nervous system primarily has to inform the animal of the con- 

 ditions, good and bad, in the environment, to correlate this information 

 and to regulate the motions so that advantage may be had of this knowl- 

 edge. These facts have determined several features of the nervous 

 system. Thus they have determined its origin in the ectoderm, the 

 outer layer of the body, which comes into relation with the external 

 world. Since this information has to be carried to internal parts, con- 

 ducting tracts or nerves have arisen, while the correlating function 

 has been localized in the body of the cells where incoming and out- 

 going tracts meet. 



Most important of the primitive functions was the determination 

 of the character of the food, which would lead to the greater aggregation 

 of the nervous tissue around the mouth. As we have seen (p. n) 

 the anlage of the central nervous system of the vertebrates occupies 

 such a position. around the blastopore, or mouth of the gastrula, in the 

 form of the neural plate. As the external surface of the body is most 

 exposed to injury, the nervous structures, with the closure of the blasto- 

 pore, have been protected by removal to a deeper position, through the 

 rolling of the plate into a tube. The closure of the blastopore brings 

 the two halves of the plate into close association with each other, making 

 it a bilateral structure. With bilaterality comes the tendency of one 

 end of the animal to take the lead, resulting in the concentration of 

 nervous and sensory structures at the anterior end, which first comes 

 in contact with foreign objects. In this way a brain has been special- 

 ized apart from the rest of the nervous system. 



With the appearance of metamerism in the mesothelium and the 

 development of muscles from the myotomes there results a serial 

 repetition of motor nerves going to these, since each muscle must have 

 its own nerve supply, while sensory nerves are the result of the sinking 

 of the neural plate to a deeper position, as the sensory organs must be 

 largely in the skin. 



