782 THE CENTRAL NERVOUS SYSTEM 



it can be gleaned from a survey of the conditions under which the 

 nervous system makes its appearance in the lowest forms of animal 

 life. In the case of unicellular organisms, such as the ameba, the ap- 

 plication of a stimulus to the surface causes a movement, because the 

 protoplasm of the organism possesses, among its other properties, those 

 of excitability, conductivity and contractility. In the case of multicel- 

 lular organisms, on the other hand, some colls are set aside and spe- 

 cialized for the assimilation of food, others for movement, others to 

 receive stimuli from the outside, and yet others to compose the tougher 

 tissues which protect the surface of the animal from injury. This loca- 

 tion of specific function in specialized groups of cells makes it necessary, 

 for the welfare of the organism as a whole, that some means of com- 

 munication should be provided between the distant parts of the animal, 

 for otherwise the cells which are occupied in absorbing food would be 

 unable to move away or be protected from harm when some destructive 

 agency approached them, and indeed the moving (muscle) cells could 

 never know when the welfare of the organism as a whole demanded that 

 they should become active. 



It is probable that, in some of the lower organisms, the messages trans- 

 mitted from one group of cells to the others are carried by chemical 

 substances present in the circulating fluid — hormones, as they are called 

 (page 729). For the quick adaptation that is necessary in the struggle 

 for existence, however, such hormones are usually too slow in bringing 

 about the response, and very early in the evolutionary scale we find that cer- 

 tain cells become differentiated for this special purpose. The cells thus 

 specialized constitute the nervous system, their differentiation, as would 

 be expected, being, however, antedated by that of the cells that form the 

 muscular tissues. In the sponrjes, for example, muscle cells become 

 developed from ameboid epithelium and from a layer underneath the 

 external epithelium. These muscle cells contract slowly so as to cause 

 opening and closing of the small mouths, or oscula, on the surface of 

 the sponge in response to movements in the sea water. They are in- 

 dependent of any nervous structures. 



In certain Ccelenterates the muscle cells respond a little more quickly 

 than in the sponges, and this greater efficiency is found to be dependent 

 upon the appearance of a localized, very primitive nervous system 1 . 

 This nervous system consists of specially modified epithelial cells, or 

 receptors, sending branches from their inner ends, which either come in con- 



tael with the muse] lis. of effectors. In the region between the receptors 



and the effectors the network at first serves merely as a structure whereby 

 the entire musculature of the animal can he brought into harmonious action 

 from a single point on the surface, as, for example, in the ease of the sea 



