Neuroanatomy — Sam L. Clark 



15 



clei" is uncertain. Visceral innervation 

 could be studied to advantage with ref- 

 erence to the type of sensation conveyed 

 by the vagus nerve, the evidence of seg- 

 mental innervation, and the course and ter- 

 mination of secondary pathways convey- 

 ing visceral sensibility. 



GRAY MATTER OF NERVOUS SYSTEM 

 In the distribution of gray matter in the 

 nervous system there are certain obvious 

 general trends evident as one examines a 

 whole series of animal forms. There is early 

 in the scheme the centralization of cell 

 bodies, the accumulation into ganglia and 

 then larger masses of central gray matter, 

 the tendency to encephalization, and the as- 

 sociated precocious development of the 

 cranial end of the nervous system. Along 

 the way an endo-skeleton develops and 

 forms an encasement for the nervous sys- 

 tem which comes to lie dorsal to the ali- 

 mentary tract instead of ventral to it, and 

 explanations of the changes are not en- 

 tirely adequate. The ingenious theory of 

 the origin of vertebrates by way of the 

 annelids and arthropods requiring that 

 the latter turn over on the back and de- 

 velop a new mouth because of the en- 

 largement of the nervous parts and the 

 associated constriction of the stomodeum 

 which remains as the hypophysis, has been 

 largely discarded for a theory of origin by 

 way of the echinoderms and hemichor- 

 dates. But even with adequate descrip- 

 tion of the stages of transition there still 

 remains the larger question of how and 

 why the nervous system increases in vol- 

 ume and complexity. The question must 

 in part be answered by morphological 

 studies. 



Even the general distribution of the 

 masses of gray matter in the nervous sys- 

 tem raises questions. The location of some 

 groups of nerve cell bodies has been theo- 

 retically explained by the principle of neu- 

 robiotaxis, but this requires substantia- 

 tion. The significance of the peripheral 

 location of autonomic ganglia, and the 

 two-neuron chain between central nervous 

 system and visceral effector, is not entirely 

 clear. There is also the constant question 



as to whether there exists a complete mech- 

 anism for local reflexes in these ganglia. 

 It also would seem to require explanation 

 that most primary afferent neurons lie out- 

 side the neuraxis, though a few are within 

 it in the mesencephalic nucleus of the 

 trigeminal nerve, and perhaps other groups 

 occur. 



Within the spinal cord and medulla the 

 general principle of segregation of motor 

 and sensory cell groups in alar and basal 

 laminae, and subdivisions of these, has 

 aided in emphasizing the location of cell 

 groups of different functions; but questions 

 remain that may be more difficult to an- 

 swer if there is strict adherence to these 

 principles of distribution. Evidence from 

 Corbin's work, for example, points to the 

 possible location of proprioceptive cells 

 for intrinsic muscles of the eye in the region 

 of the oculomotor nucleus in the basal 

 lamina. If this is established other pro- 

 prioceptive cell groups of cranial nerves 

 might be easier to locate, but even more 

 significant to the result would be the gen- 

 eral questions as to the accuracy of the 

 terms "sensory" and "motor" as applied 

 to alar and basal laminae. 



Even the reasons for segregation of the 

 large mass of gray matter into separate 

 suprasegmental structures, cerebellum, tec- 

 tum, and cerebrum are not at hand; and 

 within these the localized distribution of 

 representation in specific areas offers op- 

 portunity for fruitful investigation. It 

 would be interesting to know why there 

 is greater homolateral representation in 

 the cerebellum and a larger cross repre- 

 sentation in the cerebrum, as well as the 

 reasons for the "inverted" representation 

 in the cerebral motor and sensory areas of 

 man. A comparative study of the relative 

 position of the points in the motor area' 

 in mammals suggests that the area is 

 swung from an antero-posterior represen- 

 tation to a transverse one as the frontal 

 lobe developed. But we do not know why 

 the frontal association areas developed 

 where they did, other than that old "habit" 

 of the brain of telencephalization. 



Aside from an explanation of the rea- 

 sons involved, it would be good to have 



