PHYSIOLOGY OF NERVE TISSUE. 69 



The neuron, or neurologic unit, is histologically a nerve-cell, the 

 surface of which presents a greater or less number of processes in 

 varying degrees of differentiation. As represented in figure 7, the 

 neuron may be said to consist of: (i) The nerve-cell, neurocyte, or 

 corpus; (2) the axon, or nerve process; (3) the end tufts, or ter- 

 minal branches. Though these three main histologic features are 

 everywhere recognizable, they exhibit a variety of secondary features 

 in different situations in accordance with peculiarities of function. 

 Though the nerve-cell and the nerve-fiber are but part of the same 

 neuron, it is convenient at present to describe them separately. 



The Nerve-Cell. The nerve-cell, or body of the neuron, presents 

 a variety of shapes and sizes in different portions of the nervous 

 system. Originally ovoid in shape, it has acquired, in course of de- 

 velopment, peculiarities of form which are described as pyramidal, 

 stellate, pear-shaped, spindle-shaped, etc. The size of the cell varies 

 considerably, the smallest having a diameter of not more than -g-foft 

 of an inch, the largest not more than J^ of an inch. Each cell 

 consists of granular, striated protoplasm, containing a distinct vesicu- 

 lar nucleus and a well-defined nucleolus. A cell membrane has not 

 been observed. From the surface of the adult cell portions of the 

 protoplasm are projected in various directions, which portions, 

 rapidly dividing and subdividing, form a series of branches, termed 

 dendrites or dcndrons. In some situations the ultimate branches 

 ol the dendrites present short lateral processes, known as lateral 

 buds, or gemmules, which impart to the branches a feathery appear- 

 ance. This characteristic is common to the cells of the cortex, of 

 the cerebrum, and of the cerebellum. The ultimate branches of the 

 dendrites, though forming an intricate feltwork, never anastomose 

 with one another, nor unite with dendrites of adjoining cells. Ac- 

 cording to the number of axons, nerve-cells are classified as monax- 

 onic, diaxonic, polyaxonic: Most of the cells of the nervous system 

 of the higher vertebrates are monaxonic. In the ganglia of the pos- 

 terior or dorsal roots of the spinal and cranial nerves, however, they 

 are diaxonic. In this situation the axons, emerging from opposite 

 poles of the cell, either remain separate and pursue opposite direc- 

 tions, or unite to form a common stem, which subsequently divides 

 into two branches, which then pursue opposite directions. (See Fig. 

 7.) The nerve-cell maintains its own nutrition, and presides over 

 t.-at of the dendrites and the axon as well. If the latter be separated 

 in any part of its course from the cell, it speedily degenerates and dies. 



