TISSUES. 107 



brain or ganglia, heal by production of connective 

 tissue and accompanying scar. 



The function of the axis cylinder is to conduct 

 a nerve impulse. Physiologically such an impulse 

 travels away from the cell, but experimentally it may 

 pass in the opposite direction, as is the case when a 

 nerve is stimulated midway in its course. The axis 

 cylinder being made up of fibrils, it follows that such 

 a cylinder may conduct more than one impulse, 

 which in turn reach different centers through dif- 

 ferent collaterals. 



The function of the medullary sheath is to protect 

 and nourish the axis cylinder. Experimentally the 

 non-medullated nerve fibers will tire quicker than 

 the medullated. The nodes of Ranvier are points 

 where nourishment from the blood and lymph can 

 reach the cylinder. It is affirmed by some that the 

 endolemma is only a lymph space surrounding the 

 axis cylinder. 



The neurilemma is protective in function and gives 

 great strength to the fibers. With nerves that ter- 

 minate in muscle fibers the neurilemma is continuous 

 with the sarcolemma of the muscle. Proximally the 

 neurilemma begins where the medullary sheath takes 

 up, always a short distance from the nerve cell, which 

 leaves the axis cylinder uncovered as it emerges from 

 the cell. 



It is affirmed that the neuron represents the ele- 

 mentary unit of nerve tissue, and that neurons are 

 merely in contact with each other and not in proto- 

 plasmic continuity. This idea constitutes the neuron 

 theory. 



