Chap, xii.] THE NERVOUS SYSTEM. 393 



are the anterior neural spines ; these rub on the suc- 

 ceeding and more solid portion of the vertebral 

 column. 



CHAPTER XII. 



THE NERVOUS SYSTEM AND ORGANS OF SENSE. 



THE nervous system of an animal is the apparatus 

 by means of which it becomes acquainted with what 

 is going on around it, is enabled to distribute that 

 information throughout itself, or to bring it to some 

 central region, and to set itself in proper relation to 

 the surrounding medium. In consequence of this 

 relation to the outer world, we -find that the system 

 is, at first, superficial in position and diffused in 

 arrangement, that is to say, it at first lies in the outer 

 layer of the body, with which, indeed, it at all times 

 remains closely connected ; and that, primitively, the 

 system is more or less equally distributed throughout 

 the whole of the organism. 



As we know, the Protozoa have no definite 

 nervous system, but we have already learnt than an 

 Amoeba is so far sensitive that stimuli applied to its 

 surface are followed by changes in the disposition of 

 its protoplasm. Nor have we any knowledge of a 

 nervous system in Sponges. (See page 431.) 



In all other groups of the Metazoa we have 

 evidence of the presence of cells set apart for the 

 genera] function of informing the organism of what 

 is going on around it. 



When we inquire as to what are the essential con- 

 stituents of a nervous system, we find that they are 

 either central (ganglionic) cells or conducting 

 fibres ; and, as we advance through the scale of 

 organisation, we observe that both cells and fibres 



