HISTOLOGY OF NERVE TISSUE. 6l 



aggregate of an infinite number of neurons, each of which is histologically 

 distinct and independent. Though having a common origin, as shown by 

 embryologic investigations, they have acquired a variety of forms in different 

 parts of the nervous system in the course of development. The old concep- 

 tion that the nervous system consists of two distinct histologic elements, 

 nerve-cells and nerve-fibers, which differed not only in their mode of origin, 

 but also in their properties, their relation to each other, and their functions, 

 has been entirely disproved. 



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 terminal 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. Origi- 

 nally ovoid in shape, it has acquired, in course of development, 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 FsW of an inch, the largest not more than ^Q of an 

 inch. Each cell consists of granular, striated protoplasm, containing a distinct 

 vesicular 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 subdivid- 

 ing form a series of branches, termed dendrites or dendrons. In some situations 

 the ultimate branches of 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 cere- 

 brum, 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. According to the number of 

 axons, nerve-cells are classified as monaxonic, diaxonic, polyaxonic. Most 

 of the cells of the nervous system of the higher vertebrates are monaxonic. 

 In the ganglia of the posterior 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 



