46 



PSYCHOBIOLOGY 



of any other important function of the nerve cell. 10 In its development 

 the power of conduction, which is possessed by less-highly specialized cells 

 (and even by muscle cells which are specialized in a different direction) 

 has reached the highest degree of efficiency. Coincidentally, the nerve cell 

 has lost the power of contraction, and like muscle and other highly-special- 

 ized cells, the power of reproduction. 



A neuron as a whole can conduct in but one direction. It receives im- 

 pulses through the dendrites (of which there may be several) and sends 

 them out through the axon (each cell having one axon). 



Taking the cell-body as a center of reference, we may say that the den- 

 drites conduct in, and the axon conducts out. The only possible difference 



Fig. 32. Synaptic connections of axon-branches with cell-bodies in the cerebellum. 

 Highly magnified. (Ramon y Cajal.) b, c, axon of cell B, with branches in contact 

 with 'cells of Purkinje' in row at right of A. 



between conduction by fibers (**. e., axon and dendrites) and by the cell- 

 body, however, lies in the fact that the ' current ' or 'discharge' may be- 

 come intensified in passing through the cell-body. 



Thus, a feeble irritation carried into a cell-body, either from its den- 

 drites or from the axon branches of another cell in contact with it, may 

 emerge through its axon as an intense current. 



10 It is -possible that certain 'motor' neurons are capable of rhythmic, automatic 

 action : »'. e. that they ' discharge ' repeatedly, without external stimulation, just as 

 the cardiac muscle cells contract rhythmically without external stimulation. But 

 this has not been conclusively demonstrated. 



