I9 o GENERAL BIOLOGY 



throughout its length. Those nerves issuing through 

 openings in the base of the cranial cavity of the skull 

 are called cranial nerves, and those issuing from the inter- 

 spaces between the vertebrae are called spinal nerves. 



The nervous apparatus of the body is composed of nerve 

 cells and their processes. Where the bodies of the cells 

 predominate, as in the center of the cord and in the sur- 

 face layer of the fore part of the brain, they give the nervous 

 tissue a pale grayish cast; and where the fibres predominate, 

 the tissue appears white (the so-called "gray matter" and 

 "white matter" of the nerve centers) . We have seen a very 

 simple sort of differentiation of nerve cells with processes 

 in the hydra (fig. ioi/). And in the earthworm (fig. 109) 

 we have found them very highly differentiated. But in the 

 vertebrates the processes from nerve cells are often very 

 much longer and the interrelations between them often 

 much more complex. Each spinal nerve consists of a 

 bundle of these long processes or fibres, inclosed in a com- 

 mon sheath. 



Spinal nerves arise in pairs between the vertebras, as 

 already noted, each by two roots (which are also bundles 

 of fibres), and out upon the dorsal root, just before its 

 confluence with the ventral to form the completed nerve, 

 there occurs a little isolated cluster of nerve cells: that is, 

 a ganglion. There are other nerve cells in the organs 

 of special sense, and at the termini of sensory nerves all 

 over the surface of the body. The apparent branching 

 of the nerves is due to the division of the bundle of 

 fibres into lesser bundles, and finally into single fibres 

 that take different courses to their appropriate endings. 

 The fibres themselves are continuous, and extend from 

 cells in the cord or in ganglia, to other ganglia or to 

 peripheral parts of the body. They are individual lines 

 of nervous communication; they separate as do tele- 



