Neuroanatomy — Sam L. Clark 



13 



nervation of most regions of smooth muscle 

 is apparent. There are evidently differ- 

 ences in different areas since no one has 

 demonstrated a nerve ending for every 

 smooth muscle cell in the walls of the vis- 

 cera or blood vessels, though each cell of 

 the iris and ciliary muscles appears to have 

 its own ending. Even here the exact rela- 

 tionship of the ending to the cell is dis- 

 puted, and the relationship of the nerve 

 endings to the production of humoral sub- 

 stances is again a significant question. 



There is general agreement that auto- 

 nomic nerve fibers do not supply skeletal 

 muscle fibers with endings, in mammals at 

 least. But in the eyes of birds (and rep- 

 tiles) the intrinsic muscles are striated and 

 innervated by a two-neuron chain, the 

 postganglionic cells of which lie in the 

 ciliary ganglion. The difference in form 

 of these neurons, as compared with auto- 

 nomic cells of the sympathetic trunk gang- 

 lion, must be significant. This break in 

 the principle of the relationship of the 

 nervous system to different types of muscle, 

 to be understood, would require detailed 

 comparative studies. If such an arrange- 

 ment ever occurs in mammals perhaps it 

 would be in border zones such as in the 

 wall of the esophagus where smooth and 

 striated muscle become intermingled. 

 Closer study of such areas is indicated. 



When the inquiry into moq^hology on a 

 microscopic plane is pressed still further 

 there is much to be done to determine the 

 nature of the barrier everywhere between 

 the interior of a neuron and its environment 

 on the one hand, and between the interior 

 of cell "organs" such as nucleus, Golgi 

 apparatus, mitochondria, neurofibrils, etc., 

 and the surrounding cytoplasm on the 

 other. The thinness of the lipoid covering 

 of mitochondria makes one wonder if the 

 forces across these thin membranes are in 

 the high range of those Hardy demon- 

 strated across lipoid films in his study of 

 boundary phenomena in systems outside 

 the body. Even the diameters of various 

 cell organs and calculations of surface ten- 

 sions displayed in various cell inclusions 

 would seem to be important in the light of 

 D'Arcy Thompson's reminder that as the 



diameter of a small sphere approaches 

 molecular dimensions the surface tension 

 involved in maintaining its shape is meas- 

 ured in the range of thousands of atmo- 

 spheres of pressure. In making inquiries 

 concerning the direction of the chromatin of 

 the nucleus, the provocative findings of 

 Delbriick that the gene is very likely a 

 single large protein moleclule containing 

 only about a thousand atoms, make it de- 

 sirable to know the basis of power of this 

 potent regulator of orderliness in the ner- 

 vous system. How do such structures re- 

 duplicate themselves in cell division? Here 

 and elsewhere the use of the electron mi- 

 croscope will no doubt be of value. 



At such levels morphological details are 

 to be studied in terms of chemistry and 

 physics. Knowledge is available from the 

 use of microchemical techniques of the 

 distribution of iron and other inorganic 

 substances in nervous tissue, but the 

 method of micro-incineration has not been 

 completely exploited as a means of study- 

 ing the distribution of various elements. 

 The use of radioactive substances and the 

 method of having deposits of these show 

 their microscopic distribution by acting 

 upon a superimposed photographic emul- 

 sion promises significant results. Distri- 

 bution of enzyme systems in neurons, so 

 well begun with alkaline and acid phos- 

 phatase, offers further promise of the value 

 of microscopic morphology as a field of 

 study. The studies of Caspersson on the 

 distribution of nucleo-proteins in nerve 

 -cells have opened new fields. 



Alterations as a result of pathological 

 processes both in the chemistry of neurons 

 and their morphology is another means of 

 study needing attention. The method of 

 chromatolysis still yields results under 

 proper control and displays the cells of 

 origin of a cut nerve pathway. The changes 

 produced in the myelin sheath with degen- 

 eration have been of great value in tracing 

 pathways by specific staining of the altered 

 myelin, and the method is in constant use. 

 If one could obtain a comparable method 

 of selective staining of axis cylinders, cut 

 loose from the cell bodv and so on the 

 way to degeneration but not yet frag- 



