14 



Survey of Neurobiology 



merited, the whole detailed anatomy of the 

 nervous system could be rewritten and 

 answers given to many current questions. 

 The results of the method analyzing spe- 

 cific differences occuring in boutons term- 

 inaux in early stages of. degeneration are 

 not yet sufficiently reliable. 



The fact that certain viruses travel along 

 axis cylinders and, as shown by Goodpas- 

 ture, produce detectable changes in nuclei, 

 cell bodies, and axis cylinders of the neu- 

 rons involved has not been given sufficient 

 trial as a means of studying connections in 

 the nervous system. 



Larger problems of morphology may 

 have their answers at microscopic levels. 

 Dr. Cannon once asked, "Are there 'bushes' 

 and 'trees' among nerve cells and therefore 

 different limits to the possible growth of 

 them?" The accepted fact that nerve 

 fibers grow as the body grows has been 

 taken for granted but the question of how 

 long nerve fibers may be made to grow, or 

 how far they may be stretched, has been 

 raised by the recent successful attempts of 

 Hoen and Bering to fill gaps in damaged 

 nerves by stretching gradually the central 

 stump. 



Problems of regeneration include many 

 morphological details which have been 

 well worked out, but there are additional 

 needs. The influence of substances (such 

 as pyrogens) introduced into the body of 

 an animal with central nervous system dam- 

 age on the possible regeneration of nerve 

 fibers or the inhibition of glial scarring 

 would seem to be a promising field of 

 study. When tumors ancl new growths arise 

 there is some evidence that they may be- 

 come innervated, but the factors in this 

 are not clearly known. Transplantation of 

 mature nerve cells both of cerebrospinal 

 and autonomic ganglia into another part of 

 the animal has been accomplished with 

 survival of some cells for long periods of 

 time, but only recently has evidence been 

 produced that mature nerve cells can be 

 successfully transplanted into tissue cul- 

 tures, where they may even undergo 

 mitosis. Such an observation indicates 

 need of further study of the binucleate 



cells of ganglia and the possibility of multi- 

 plication of nerve cells. 



CEREBROSPINAL NERVES 



The problem of segmental innervation 

 of the skin and muscles appears in need of 

 clarification. The need has been empha- 

 sized by the variations in mapping of der- 

 matomes as revealed by measurements of 

 skin resistance following sympathetic de- 

 nervation, in contrast to maps based on 

 cases of herpes zoster, spinal nerve root 

 section, and deeper segmental representa- 

 tion as indicated by injections of saline 

 into interspinous ligaments. 



The composition of peripheral nerves is 

 rather well established but the question 

 continually arises as to whether the dorsal 

 roots are purely sensory and the ventral 

 ones purely motor. Suggestive evidence 

 that Bell's law does not hold for dorsal 

 roots appears from physiological experi- 

 ments, particularly from those recording 

 action potentials; and for the ventral roots 

 from studies on pain in tabes and in indi- 

 viduals with dorsal roots cut. However, 

 the morphological proof in the case of 

 both ventral and dorsal roots is incomplete. 



The cranial nerves pose a number of 

 problems, prominent among which are 

 questions arising out of the classification 

 of functional components so well estab- 

 lished by the work of American compara- 

 tive neurologists. There is need for con- 

 tinuing and modifying this classification 

 by close study of comparative anatomical 

 details. Conflicts exist within it, for ex- 

 ample, classification as "general somatic 

 afi^erent" of the proprioceptive fibers sup- 

 plying muscle sense organs in muscles of 

 mastication which are supplied by motor 

 fibers classed as "special visceral efferent." 

 While this and other problems are partly 

 academic ones due to the use of either 

 one of two separate criteria for classifica- 

 tion, there are additional problems to be 

 solved in the location of cell groups con- 

 cerned with specific functions. For ex- 

 ample, little is known of the location of 

 cells giving rise to the proprioceptive fibers 

 of the cranial nerves, except those of the 

 trigeminal. Location of the "salivatory nu- 



