HISTOLOGY. 19 



Nervous Tissues. 



Nervous tissue has for its function the correlation of the animal with 

 its environment. In order to accomplish this it must provide for the 

 recognition of stimuli from without, the inauguration of other impulses 

 within itself and the transfer of both to other parts. The essential 

 constituent of the tissue is the nerve cell, ganglion cell or neuron, 

 to which are added others of a supportive (glia cells) or nutritive 

 character. As the parts to be connected by the nervous tissue are often 

 remote from each other the neuron is not compact like most other cells, 

 but gives off long processes from the central mass, these processes differ- 

 ing in their terminations. Some end in places where they can only 



FIG. 12. Various kinds of nerve cells. A, multipolar cells; B, portion of nerve fibre 

 with sheaths; C, unipolar cell; D, pyramidal cell; a, axon; c, collateral; d, dendrites; cb, 

 cell body; m, medullary sheath; n, nucleus of cell of Schwann's sheath; s, sheath of Schwann; 

 t, telodendron. 



receive stimuli, others where the stimuli can only cause parts to act. 

 Thus the processes are physiologically divisible into afferent and 

 efferent tracts, the body of the cell being the place for the regulation and 

 correlation of the impulses, and apparently in many cells for the inau- 

 guration of new impulses. 



A nerve cell (fig. 12) is uni-, bi- or multipolar accordingly as it has 

 one, two or more of these processes. In the case of unipolar cells (C) 

 the single process sooner or later divides, so that the cell in reality is at 

 least bipolar. At the ends the processes may either break up in minute 

 twigs (dendrites, d} or may end, as in muscles and sense organs, in 

 special end organs. The part connecting the efferent termination and 

 the central cell body is called the axon (a). Axons and cell bodies are 

 gray in color, but usually the axons are surrounded by a medullary 

 sheath (m) of a peculiar white substance (myelin) rich in fat, which 



