72 



NERVOUS TISSUE. 



Cells which have but one axis-cylinder process are mononeuric ; 

 those having two such processes are dineuric ; and trineuric is 

 applied to those having three. Most nerve-cells are mononeuric. 

 Ganglia. A ganglion is a collection or group of nerve-cells. 

 These occur upon the posterior roots of the spinal nerves (Fig. 

 78), upon some of the cranial nerves, and in connection with the 

 sympathetic nervous system. In these structures the cells have 

 a nucleated sheath continuous with that of the nerve-fibers con- 

 nected with them. From each cell in the ganglion, upon the roots 

 of the spinal cord, and among the cranial nerves is given off but 

 one process, the axis-cylinder process. Passing in a convoluted 

 form from the cell, this process, before it leaves the ganglion, 

 divides into two, one going to the nerve-center, the other to the 



periphery. From this descrip- 

 tion it will be seen that these 

 cells have no dendrons. 



In the cells of the sympathetic 

 ganglion, besides the axis-cylin- 

 der process, there are also sev- 

 eral dendrons. 



Protoplasmic Process. Unlike 

 the axis-cylinder process, this va- 

 riety is characterized by its fre- 

 quent branching. The larger 

 branches are called dendrons, 

 and the finer ones dendrites. 



The idea that the axis-cylin- 

 der process alone conveys ner- 

 vous impulses, and that the den- 

 drons and dendrites are nutritive organs exclusively, is at the 

 present time replaced by the belief that nervous impulses also 

 travel along the branches of the protoplasmic process. The 

 anatomic fact that the fibrils of the axis-cylinder have been 

 traced through the body of the cell into the dendrons, seems to 

 substantiate this theory. 



It is a most important fact that the nerve-unit, or the " neu- 

 rone" of some writers that is, the nerve-cell and its branches does 

 not anastomose or join with any other nerve-unit, but the terminal 

 twigs or arborizations of one intertwine with those of another, and 

 nerve-impulses may thus pass from one to the other. This inter- 

 twining is called synapse, a word literally meaning a clasping. 

 This subject will be again referred to when the physiology of 

 nerves is discussed. 



Netirpglia (Fig. 79). This is sometimes spoken of as a con- 

 nective tissue, but it is in structure unlike connective tissue as we 

 have studied it. It is also unlike it chemically, consisting of 



Nucleus. 



FIG. 78. Ganglion-cell with a pro- 

 cess dividing at a (T-shaped process) ; 

 from a spinal ganglion of the frog ; 

 X230 (Bohm and Davidoff). 



