Ja.m auy U, 1905.] 



SCIENCE. 



23 



rones C()nnc'ctin<; the skin with the central 

 nervous system, and to see how this num- 

 ber varies in the dilt'erent species of mam- 

 mals similar in form but unlilie in size. 

 There is only (.ne animal, the white rat, on 

 which as yet such stiidies have been made, 

 so that the whole field lies practicality open. 

 Should we be able to get good numerical 

 evidence in favor of the view that under 

 the conditions named above, the afferent 

 system could be taken as an index of the 

 size of the brain, it would show us at once 

 that in the laying down of the nervous sys- 

 tem certain proportions Avere rather rigidly 

 observed, and bring us to the next step, 

 namely, the determination of the influences 

 which control those proportions and the 

 possibility of effecting an alteration in 

 them. In the meantime, there is every 

 reason to prepare for the application of 

 these results to man, and althoiigh the pro- 

 gram here is simple enough to state, it will 

 involve great labor to carry it through. 



So far as the numerical relations in man 

 are concerned, we have, through the work 

 of Dr. Helen Thompson,* an excellent esti- 

 mate of the number of nerve cell bodies in 

 the human cortex, and through that of Dr. 

 Ingbert,t a reliable count of the number 

 of medullated nerve fibers in the dorsal 

 and ventral roots of the thirty-one pairs 

 of spinal nerves of a man at maturity. It 

 is easy to see, however, that we must get 

 some notion of the amount of individual 

 variation to which these relations are sub- 

 ject within the limits of one race and one 

 sex before it is desirable to attempt to 

 learn whether the difference in race or sex 

 here plays an important role. It is to be 

 anticipated, however, that the differences 

 dependent upon race and sex will be com- 

 paratively slight, and especially so when 

 conti-asted with the diflPerenees which we 



* Thompson, Journ. of Comp. Ncm-oL, 1899. 

 t I'lgbert, Journ. of Comp. Neurol., 190.3 and 

 1904. 



may anticipate as existing between the 

 adult and the child at birth. . This aspect 

 of the problem illustrates, in a concrete 

 form, the sort of question which is raised 

 by the anatomical study of the body dur- 

 ing the period of growth. The einbryolo- 

 gists have worked out the formation and 

 early developmental history of the various 

 organs and parts of the human body, but 

 the study of the later foetal stages have 

 been blocked by the scarcity of material, 

 and the inconvenience of dealing with it. 

 On the individual at birth, Ave have again 

 more extensive observations, but for the 

 period comprised between the first two 

 years of life and the age of twenty our 

 information is again scanty. The loAver 

 death rate during this part of the life cycle, 

 as Avell as social influences, combine to keep 

 material between the.se ages out of the dis- 

 secting room. Here is an important part 

 in the life history of man Avhich needs to 

 lie investigated along many lines, and dur- 

 ing which it is most desirable to have a 

 record of the changes in the nervous sys- 

 tem expressed in quantitative terms. In 

 the general problem Avhich is here under 

 discussion, our next step Avould be to emi- 

 merate in man at birth the medullated 

 nerve fibers in the roots of the .spinal 

 nerves. Such an enumeration will prob- 

 ably show us betAveen birth and maturity 

 a very large addition to the number of 

 these fibers, but we still have to determine 

 at Avhat portion of the period, and accord- 

 ing to Avhat laws, this addition takes place. 

 At this point our observations on animals 

 will assist us, and we should certainly look 

 for the occurrence of greatest addition dur- 

 ing the earlier part of the growing period. 



Let us assume then that Ave have ob- 

 tained results AA'hich show us the normal 

 development of this portion of the nervous 

 system between birth and maturity. These 

 observations could be used as a standard. 

 Once possessed of such a standard, we are 



