THE NERVOUS SYSTEM.— GENERAL CONSIDER. 

 ATIONS. 



Since in the higher vertebrates the nervous system is domi- 

 nant, regulating apparently every process in the organism, it 

 will be well before proceeding further to treat of some of its 

 functions in a general way to a greater extent than we have yet 

 done. 



Manifestly, it must be highly important that an animal shall 

 be able to place itself so in relation to its surroundings that it 

 may adapt itself to them. Prominent among these adaptations 

 are certain movements by which food is secured and dangers 

 avoided. The movements having a central origin, a peripheral 

 mechanism of some kind must exist so as to place the centers 

 in connection with the outer world. Passing by the evolution 

 of the nervous system for the present, it is found that in verte- 

 brates generally there is externally a modification of the epi- 

 thelial covering of the body (end-organ) in which a nerve ter- 

 minates, which latter may be traced to a cell or cells removed 

 from the surface (center), and from which in most cases other 

 nerves proceed. 



The nervous system, we may remind the student, consists in 

 vertebrates of centers in which nerve-cells abound, united by 

 nerve-flbers and by the most delicate form of connective tissue 

 known, in connection with which there are incased strands 

 of protoplasm or nerves as outgrowths. The main centers are, 

 of couj?se, aggregated in the brain and spinal cord. 



It is possible to conceive of the work of a nervous system 

 carried on by a single cell and an afferent and efferent nerve ; 

 but inasmuch as such an arrangement would imply that the 

 central cell should act the paii of both receiving and origi- 

 nating impulses (except it were a mere conductor, in which case 

 there would be no advantage whatever in the existence of a cell 

 at all), according to the principle of the physiological division 



