766 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



implies at least four different operations — the conduction of afferent 

 impulses, the reception and conversion of these afferent into efferent 

 impulses, and the liberation again and conduction of efferent impulses. 



In the scheme of specialization of function it would only be expected 

 that for the division of labor we should also find that these operations 

 become separated and located in different structures. We find, therefore, 

 the nervous system, in accordance with this purpose, divided into organs 

 of conduction and organs of receiving and liberating nervous impulses. 



The organs for transmitting nerve impulses constitute the nerves. 

 The organs for modifying and liberating nerve impulses are found in the 

 ganglia or nerve-cells of the nervous system. 



The scheme of the nervous system, therefore, implies the presence 

 on the periphery of a receptive organ for receiving external impressions. 

 Such an organ is represented by the terminal filaments of the sensory 

 nerves, or, rather, the nerves of general or special sense. It implies a 

 means of communication between this external receptive organ and the 

 nervous ganglia at a distance, the latter possessing the power of receiv- 

 ing the impressions transmitted from the exterior through the afferent 

 sensory nerves. Such a receptive ganglion is again in' connection with a 

 cell or collection of cells in which the automatic powers are especially 

 developed, and which, therefore, modify the impressions coming from 

 without and convert them into efferent impressions. The latter are con- 

 ducted from the centre through the efferent or motor nerves to various 

 peripheral organs, whether to the terminal plates in the muscular tissue 

 or to glands, blood-vessels, or the other structures of the animal body. 



Like all other organic systems, the nervous sj^stem becomes more 

 complicated and diversified, reaching a higher stage of perfection in pass- 

 ing from the lowest forms of animal life, in which it first appears, to the 

 higher examples of the animal series. 



In the protozoa, the lowest subdivision of the animal kingdom, a 

 nervous system in the sense in which we have described it, as constituted 

 of nerve-cells and nerve-fibres, is entirely absent. The undifferentiated 

 protoplasm fulfills all the purposes of the nervous system as demanded 

 by the needs of such an organization. 



In the animals belonging to the group of infusoria, where we find the 

 first appearance of the development of organs as seen in the contractile 

 cilia as organs of locomotion, the nervous system has not appeared, the 

 movement of such organs being dependent simply upon the automatic 

 properties of the undifferentiated protoplasm ; and we find as an illus- 

 tration of this that even in animals higher in the scale, where ciliated 

 organs are commonly found, that such are independent of the nervous 

 system. 



In the star-fish is found the first clear evidence of nerve fibres and 



