OF THE NERVOUS SYSTEM. 373 



nervous matter owes the ashy or " cineritious" color from which it 

 derives its name. The cells composing it vary in size, according 

 to Kolliker, from 4^0 to 3^5 of an inch. The largest of them have 

 a very distinct nucleus and nucleolus. (Fig. 122.) Many of them 

 are provided with long processes or projections, which are sometimes 

 divided into two or three smaller branches. These cells are inter- 

 mingled, in all the collections of gray matter, with nervous filaments, 

 and are entangled with their extremities in such a manner that it 

 is exceedingly difficult to ascertain the exact nature of the anato- 

 mical relations existing between them. It is certain that in some 

 instances the slender processes running out from the nervous vesi- 

 cles become at last continuous with the filaments; but it is not 

 known whether this be the case in all or even in a majority of 

 instances. The extremities of the filaments, however, are at all 

 events brought into very close relation with the vesicles or cells of 

 the gray matter. 



Every collection of gray matter, whatever be its situation or 

 relative size in the nervous system, is called a ganglion or nervous 

 centre. Its function is to receive impressions conveyed to it by the 

 nervous filaments, and to send out by them impulses which are to 

 be transmitted to distant organs. The ganglia, therefore, originate 

 nervous power, so to speak ; while the filaments and the nerves 

 only transmit it. Now we shall find that, in the structure of every 

 nervous system, the ganglia are connected, first with the different 

 organs, by bundles of filaments which 

 are called nerves ; and secondly with Fig- 123. 



each other, by other bundles which 

 are termed commissures. The entire 

 system is accordingly made up of 

 ganglia, nerves, and commissures. 



The simplest form of nervous 

 system is probably that found in 

 the five-rayed starfish. This animal 

 belongs to the type known as radiata; 

 that is, animals whose organs radiate 

 from a central point, so as to form a 

 circular series of similar parts, each 

 organ being repeated at different NERVOUS SYSTEM OF STARFISH. 

 points of the circumference. The 



starfish (Fig. 123) consists of a central mass, with five arms or 

 limbs radiating from it. In the centre is the mouth, and immedi- 



