INTRODUCTION 



the spinal cord the reverse is the case, a central column of 

 gray matter being surrounded by white matter (Figs. 23 



and 24). 

 The gan- 

 glia are 

 composed 



almost Fig. 24. Cross see- 

 wholly of tjon of spinal cord, 

 showing arrange- 

 graymat- ment of gray and 

 white matter. 



29. Nerve CeUs. Gray 

 nervous matter is made 



.7 ^""^ up mainly of nerve cells 



Fig. 23. Cross section of the hemispheres 



of the brain, showing arrangement of (Fig. 25). These vary 

 gray and white matter. much in gize and ghape> 



Each sends out one or more branches, or processes, one 

 of which forms the central core of a nerve fiber and 

 is called the axis cylinder process. A ganglion is a col- 

 lection of nerve cells imbedded in nerve fibers. 



30. Nerve Fibers. Every nerve fiber has connection 

 with at least one nerve cell, for its central strand, called 



the axis cylinder, is al- 

 ways the axis cylinder 

 process of a nerve cell 

 (Fig. 26). This is the 

 essential, indispensable 

 part of the nerve fiber. 

 Surrounding the axis cyl- 

 inder there is usually a 

 layer of white, oily mat- 

 Fig. 25. -Nerve cells (mitral) from the ter called t he medullary 

 olfactory bulb. , , , . , 



sheath, and outside of 



that is a thin inclosing membrane called the neurilemma. 



