66 



NORMAL HISTOLOGY. 



Dendrites 



Arborization 



of axone 



FIG. 85. Diagram of nerve-cell of 

 type II, in which axone is not pro- 

 longed as nerve-fibre. 



similar to the dendrites. Neurones are divided according to the distribution 

 of their axones into two classes. In those of the first class, known as type 1 

 cc//s, the axone is continued as a nerve-fibre and is, therefore, relatively long. 



Soon after leaving the cell-body (nerve-cell), 

 such axones give off delicate lateral processes, 

 the collaterals, which, after a longer or shorter 

 course,* break up into arborizations that end in 

 relation with other and often remote neurones. 

 The neurones composing the second and much 

 less frequent class, type II cells, possess short 

 axones tha.t are not continued as nerve-fibres, but 

 almost immediately break up into complex end- 

 arborizations or neuropodia, limited to the gray 

 matter. 



The nerve-cells, as the cell-bodies of the 

 neurones commonly are called, are in general 

 relatively large elements, those in the anterior 

 horns of the spinal cord measuring from 70-150 

 /j.. They possess a large spherical nucleus, poor 

 in chromatin but usually provided with a con- 

 spicuous nucleolus. Their cytoplasm varies in 

 appearance with the method of fixation and 

 staining to such an extent, that much uncertainty 

 exists as to the relation of many described details 

 to the actual structure of the cells. It is probable, 

 however, that the cell-body of the neurone consists of a ground- substance, 

 homogeneous or finely granular, in which fine neurofibrih and masses of 

 chromatophilic granules are embedded; in addi- 

 tion a variable amount of brown or blackish 

 pigment is usually present in the vicinity of 

 the nucleus. The neurofibrils are continued 

 into all the processes as far as the terminal 

 arborizations and form a network surround- 

 ing the nucleus that is enclosed by a superficial 

 one. After special staining with methylene 

 blue, the chromatophilic granules appear deeply 

 colored and grouped in variable masses, known 

 as Nissl bodies, which occupy the interstices of 

 the fibrillar reticulum. Collectively the gran- 

 ules of "stainable substance" constitute the 

 tigroid substance and are most conspicuous in 

 the neighborhood of the nucleus and least so 

 at the periphery of the cell. They are continued 

 into the dendrites as elongated flakes that finally 

 resolve into scattered granules along the proc- 

 esses. The axone, on the contrary, does not 

 contain Nissl bodies, and usually joins the cell- 

 body at an area free from the stainable sub- 

 stance, the process usually arising from a slight 

 elevation known as the implantation cone. 

 Exceptionally, the axone arises from one of the dendrites, either at its 

 base or at some distance from the cell-body. Owing to the size of the cells, 

 little more than the stumps of the processes are ordinarily seen in sections. 



FIG. 86. Semidiagrammatic repre- 

 sentation of structure of neurone ; a, 

 axone. 



