THE NERVOUS TISSUES. 



branches of the dendrites form a dense feltwork, which, together 

 with the cell-bodies of the neurones and with other elements to be 

 described later, constitute the gray substance (gray matter) of the 

 brain and spinal cord. 



All neurones, with possibly a few exceptions, possess only a 

 single neuraxis. Neurones without a neuraxis have never been 

 found in vertebrates. The neuraxis usually arises from a cone- 

 shaped extension of the cell-body free from chromatophile granules, 

 the implantation cone or axone hillock, more rarely from the base of 

 one of its dendrites, or from a dendrite at some distance from the cell- 

 body. Its most important characteristics are its smooth and regular 

 contour and its uniform diameter. At some distance from the cell- 

 body, usually near its termination, now and then in its course, a 

 neuraxis may divide into two 

 equal parts. Golgi (94) called 

 attention to the fact that the neu- 

 raxes of certain neurones (Pur- 

 kinje's cells in the cerebellum, 

 pyramidal cells of the cerebral 

 cortex, and certain cells of the 

 spinal cord) give off lateral pro- 

 cesses, the collateral branches. 



Fig. 108. Chromatophile granules of 

 a ganglion cell from the Gasserian ganglion 

 of a teleost : a, Nucleus ; b, implantation 



Fig. 109. Nerve-cell from the ante- 

 rior horn of the spinal cord of an ox, 

 showing coarse chromatophile flakes. 



Two types of cell are recognized according to the disposition of 

 their neuraxes : In the first the neuraxis is continued as a nerve- 

 fiber ; in the seoond and rarer type it does not long preserve its 

 independence, nor is it continued as a nerve-fiber, but soon breaks 

 up into a complicated arborization, the neuropodia of Kolliker (93). 

 The latter type of cell occurs in the cortex of the cerebrum and 

 cerebellum and in the gray matter of the spinal cord. The cells of 

 the two types can be simply described as having long (type I) or 

 short, branched neuraxes (type II). The neuraxes of the cells of 

 type I possess the collateral branches which end in small branching 

 tufts. 



In its simplest form, a neurone consists of a cell-body and a neu- 

 raxis with its telodendrion. In more complicated types one or several 



