306 COORDINATION AND SENSATION 



body may continue to live. In addition to this the cell- 

 body probably reenforces the nervous impulse. 



The dendrites serve two purposes : first, they extend 

 the surface of the cell-body, thereby enabling it to absorb 

 a greater amount of nourishment from the surrounding 

 lymph ; second, they act as receivers of stimuli from 

 other neurons. The same impulse does not pass from one 

 neuron to another. An impulse in one neuron, however, 

 is able to excite the neuron with which it makes an end- 

 to-end connection, so that a series of impulses is produced 

 along a given nerve path (Fig. 129). 



The special function of the axon is to transmit the impulse. 

 By its length, structure, and property of conductivity it is 

 especially adapted to this purpose. The axis cylinder, 

 however, is the only part of the axon concerned in the 

 transmission. The primitive sheath and the medullary 

 layer protect the axis cylinder, and, according to some au- 

 thorities, serve to insulate it. The medullary sheath may 

 also aid in the nourishment of the axis cylinder. 



Nerve Stimuli. While the properties of irritability and 

 conductivity supply a necessary cause for the production 

 and transmission of nervous impulses, these alone are not 

 sufficient to account for their origin. An additional cause 

 is necessary a force not found in the nerve protoplasm, 

 but one which, by its action on the protoplasm, makes it 

 produce the impulse. In this respect, the neuron does 

 not differ essentially from the cell of a muscle. Just as 

 the muscle cell requires a stimulus to make it contract, so 

 does the neuron require a stimulus to start the impulse. 

 Hence, in accounting for the activities of the body, it is not 

 sufficient to say they are caused by nervous impulses. We 

 must also investigate the nerve stimuli the means through 

 which the nervous impulses are started. Most of these 



