80 NORMAL HISTOLOGY. 



Nervous tissue is composed of cells, ganglia, and nerve fibres. 

 A nerve cell is devoid of a cell-wall; its nucleus has a prominent 

 nucleolus, and its protoplasm sometimes contains yellowish gran- 

 ules of pigment. The cells are the most abundant in nerve centers 

 the brain, spinal cord, and ganglia. The primitive nerve cells are 

 called neurollasts, and in the course of their development, by send- 

 ing out certain processes known as axis cylinders and dendrites, pro- 

 duce the different forms described as the dendron, neuro-dendron, 

 unipolar, bipolar, and multipolar cells, and first-type and second- 

 type cells. An axis cylinder is a slender protoplasmic growth from 

 the pointed end of a neuroblast. The dendrites are protoplasmic 

 growths which arise from other parts of the cell. The axis cylinder 

 attains considerable length, often as much as a meter; the dendrites 

 are short and slender. The axis cylinder gives off a few lateral 

 processes called collaterals; the dendrites branch dichotomously, 

 forming a dense network. The axis cylinder functions as a nerve 

 fibre; the dendrites possibly serve as a supporting framework for 

 the neuroplasm, and are supposed to be continuous with the ulti- 

 mate fibrils. A nerve cell with its axis cylinder is a neuron; a cell 

 with dendrites and an axis cylinder bearing collaterals is a neuro- 

 dendron, Cells are named also from the number of processes they 

 bear unipolar, one process ; bipolar, two processes ; multipolar, sev- 

 eral processes. A first-type cell is one which has a long axis cylinder 

 that becomes a medullated nerve fibre. A cell of the second-type has 

 a short axis cylinder which divides and subdivides. The neuroglia- 

 cells are small bodies which give off a multitude of fibrils forming 

 a supporting network, serving as connective tissue elements, and 

 holding together the delicate structures which enter into nerve cen- 

 ters. They are called glia cells, and the reticulum produced by 

 them is neuroglia. 



Ganglia are nerve centers consisting of groups of cells and fibres. 

 Some of the fibres which enter the ganglion terminate in its cells, 

 while others pass through to more distant points. The whole gan- 

 glion is invested with a connective tissue sheath, and each fibre is 

 enclosed with an endoneurium, which is continuous with the cap- 

 sule with which the fibre terminates. The brain and spinal cord 

 may be considered as groups of large ganglia. 



