438 STUDIES IN ADVANCED PHYSIOLOGY. 



minded of the innumerable number of nerve cells compos- 

 ing the nervous system, of their intimate inter-relation, and 

 yet at the same time of the definite way in. which they are 

 connected by their fibers, one can appreciate the precision 

 with which these primitive neuroblasts must have distrib- 

 uted themselves while determining their direction of growth. 

 In fact, there are not lacking physiologists who believe that 

 forms of nervous derangement, from insanity downwards, 

 are due to possible accidental misplacements and malforma- 

 tions of these primitive neuroblasts. 



A second interesting point concerning these neuroblasts is 

 that they do not increase in number after about the third or 

 fourth month of fcetal life, and that the entire development 

 of the nervous system following that is due to an expan- 

 sion of already formed neurons and the establishment of 

 more and more complicated channels of connection. 



Possibly the most important fact from a teacher's stand- 

 point to remember, is that the development of these channels 

 of association, these anatomical connections between differ- 

 ent neurons, is not completed until up to and even past 

 maturity, and explains the physiological impossibility for 

 the existence of nervous or mental faculties in the young 

 which later on will naturally appear. 



The Regeneration. 



When a nerve is cut, the end of the nerve fiber severed 

 from the cell body to which it belonged disintegrates, and 

 a new nerve fiber to innervate the place of the old arises as 

 an outgrowth from the stump of the end in connection with 

 the cell. If, however, the severed ends of such a nerve be 

 connected they seem soon to have grown together. This 

 growing together does not, however, affect the axis-cylin- 

 der, for the axis-cylinder grows down through the coats of 

 the old nerve not unlike the growing of a rootlet through 

 the soil. In this growth it is no doubt guided by the path 

 of the old fiber itself, with the medullary coat and primitive 

 sheath of which, contact seems to have been made. 



