296 G. E. COGHILL 



4. The development of function in the neurone and in the reflex arc 



It has already been pointed out (Paper I) that the giant gan- 

 ghon cells of the spinal cord on the one hand and the motor 

 neurones on the other have their connections made with their 

 respective end-organs for a considerable time before they become 

 integral parts of a functional reflex arc, and that, for this reason, 

 these neurones can not be used as a standard by which to judge 

 the structural changes in the neurone that mark the beginning 

 of nervous function. Now we find that the cranial afferent 

 nerves also are in the earlier stages developed in a degree that 

 seems out of proportion to the part that they can be demon- 

 strated to have in reflexes. We have noted, for example, that, 

 in the coiled reaction stage, the trigeminal nerve has a very gen- 

 eral distribution to the skin and that it has root connection with 

 the brain from a much earlier period, whereas it becomes an 

 important factor in behavior only about the time the embryo 

 begins to swim. The neurones of this nerve must for a con- 

 siderable time receive stimuli and conduct impulses that are 

 never discharged through any reflex arc to the effectors. The 

 root fibers of the visceral sensory system also are well developed 

 and the fasciculus solitarius is extensively developed for many 

 days before the neurones of this system are subjected to physio- 

 logical stimulation by the development of the mouth. None of 

 these neurones, therefore, can give us a clue as to when they 

 take on nervous functions. 



Our observations upon the development of the optic nerve in 

 relation to the function of the eye show that the same uncertainty 

 is attached to the neurones of the second or higher order in the 

 arc. In this case the retinal ganglion cells are highly differen- 

 tiated and can, under very strong stimulation of the retina, 

 become an integral part of a reflex arc at a time when the more 

 peripheral sensory elements of the arc are in an exceedingly 

 embryonic condition. The high threshold of the optic reflex 

 must then be due to the undeveloped condition of the retina 

 whereas the ganglion cells, optic nerve fibers, and the central 

 neurones reaching caudad from the diencephalon to the lower 



