NEURONS 225 



portion of the spinal cord, where it is seen to be an out- 

 growth from a comparatively large cell. This cell, and 

 not its branch, the axis cylinder, is the real centre of 

 activity, as is shown by the fact that when the axis 

 cylinder is cut off from the cell it dies, whereas the cell 

 lives on and may even produce a new and complete axis 

 cylinder. 



Neurons. From this nerve cell there are also tree- 

 like branches which serve to bring the cell into close 

 relation with other cells. Each cell and its branches, 

 including the axis cylinder, form one unit of the nervous 

 system, a neuron, of which there are many millions in 

 the brain and spinal cord. 



Motor neurons. The neurons which terminate in the 

 cells of muscles or glands, control their activity. The 

 contraction of every muscle, the secretion of every gland, 

 is directly due to the stimulations developed in the large 

 central cells of the neurons. These stimulations, or 

 nervous impulses, are transmitted along the axis cylin- 

 ders of the neurons to the muscles or glands. The neu- 

 rons are therefore called motor neurons, since upon them 

 all activity depends. If motor neurons are injured, 

 as for example by cutting the ventral root of a spinal 

 nerve, the cells of the muscle or gland supplied by their 

 fibres become paralyzed. In. other words, the cells are 

 powerless to act, because they have ceased to receive 

 nervous impulses. Unless the motor neurons recover so 

 that communication with the cells is restored, the cells 

 of the muscle or gland themselves waste away, since with- 

 out the nerve stimulation they are unable even to take 

 the food which they need from the blood. 



Course of a sensory nerve. If, instead of selecting 

 an axis cylinder going to a muscle, we had chosen an axis 

 cylinder from the cells in the nearby skin and had traced 



