NERVE TISSUES 



175 



cell, which may be a nerve cell, a muscle cell, or a cell of some other kind 

 (Fig. 155, B, etc.). That the perceptory end-organs and the discharging 

 end-organs are specific and necessary structures is proved by the fact 

 that the cell cannot operate without them or when they are injured 

 or diseased. One or both of them can be regenerated in some ani- 

 mals. Such a cell with its processes and end-organs is known as a 

 neuron. 



The nerve cell, which is usually large and well developed, may have 

 a great variety of forms. It may be compact and but little elongated 

 (Fig. 155, A}, as are many surface nerve cells used to perceive mechanical 



Ilil 1 111 



FIG. 155. Diagrams of different kinds of nerve cells. External arrows point to receiving or 

 perceptory surface; internal arrows show discharging or stimulatory surfaces of cells. A, a 

 nerve cell with no process; B, the same with its discharging surface or organ on a process; 

 C, both end-organs on processes; D, the same with impulse- path independent of the cell 

 body ; E, multiple perceptory organs ; F, both end-organs multiple. 



stimuli. As it is primarily intended, however, for communication be- 

 tween more or less widely separated parts of the body, it is almost always 

 extended by means of drawn-out processes of the cytoplasm of its cell 

 body (Fig. 155, B, C, D, E, F). The perceptory and stimulating sur- 

 faces of the cell are, of course, placed at the ends of the processes in order 

 that they may be next to the points to be communicated with. There 

 may be one process, placed either at the perceptory pole of the cell (Fig. 

 155, A) or at the discharging end (Fig. 155, C). Oftener there are two 

 processes of unequal length, one placed at each pole. The poles and 

 processes often are both on one side of the cell (Fig. 155, D). The pro- 

 cesses are sometimes single but often multiple at one end or the other. 



