VARIOUS KINDS OF CELLS: TISSUES 27 



In order that the impressions which reach the sensory cells 

 may affect the animals' behaviour, they must be transmitted to 

 the contractile and secretory cells by which that behaviour is 

 manifested. With doubtful exceptions this transmission is 

 never direct, but the impressions received by many sense cells 

 are gathered into a central cell termed a nerve cell or neuron, 

 and from it are transmitted to the glandular and contractile 

 cells. A nerve cell has a receiving and a transmitting end. 

 The receiving end usually consists of a group of root-like out- 

 growths termed receptive dendrites, which come into close 

 contact with the transmitting end of another nerve cell or with 

 the base of a sense cell. The transmitting end is a long fibre 



FIG. 5. Diagrammatic figure of a neuron or nerve cell in con- 

 nection with a simple muscle cell : ax, axon ; dend. r, re- 

 ceptive dendrites ; dend. t, terminal dendrites ; m, muscle 

 cell. 



termed an axon, which eventually ends in a tuft of branches, 

 called terminal dendrites, which are in close contact with the 

 receiving end of another nerve cell, or in contact with a muscle 

 cell or a gland cell. Now the axon may branch, and each branch 

 terminates in the same manner as the main stem ; the dendrites 

 of the receptive end of the nerve cell may also be in contact 

 with many sense cells. We thus see that the great function of 

 a nerve cell is to add together impressions received by many 

 sense cells and to distribute them to definite contractile and 

 glandular cells. In this way it comes about that the animaPs 

 response to irritation does not consist of a group of isolated 

 jerks, as it would if each sense cell communicated its impulse 

 directly to a muscular cell, but takes the form of an orderly 

 reaction of the whole body designed either to bring it nearer 



