720 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



where the male as a rule has the greater body-weight, it is to be anticipated that 

 a similar difference in the weight of the brain will be shown in other groups. 



Among individuals of the same species, but of different races or of different 

 lengths and weights, the law holds good that the larger races have the heavier 

 brains, as do the larger and heavier individuals. Here, as in the case of man, 

 it is always assumed that the differences in body-weight are mainly correlated 

 with the active tissues like muscle, and not with fat. As to the loss of the 

 brain in weight after maturity, observations on animals are scanty, but point 

 to decrease in weight toward the natural close of life. 



Interpretation of Brain-weight. In the absence of fuller data the 

 explanation of the series of differences just mentioned is, in a very high degree, 

 tentative. The loss of weight in advanced years appears to be due to a gen- 

 eral atrophy of the nerve-elements. The greater brain-weight associated with 

 greater stature appears to depend on the variations in the size of the elements 

 rather than in their number, and, so far as can be seen, the distinction accord- 

 ing to sex is susceptible of a similar explanation. 



The fact that the difference in brain-weight between the two sexes more 

 probably depends upon a difference in the size of individual elements than 

 upon a difference in the number of these elements is strongly suggested by 

 the following considerations : The microcephalic brains, constituting one group 

 which always appears in long series of records, belong to individuals whose 

 intelligence is very limited or to those to whom the functions necessary to mere 

 existence are just possible. In this latter class we have presumptively arrived 

 at a brain in which the functional elements are reduced to the lowest number 

 compatible with life. . 



Subjoined is a table giving the average weights of microcephalic brains for 

 the two sexes, the observations being divided into three groups. In each of the 

 groups taken the average weight for the females is less than that for the males : 



The Weight of the Brain in Microcephalies (condensed from Marchand). 1 



Group. 241-500 grams. 501-800 grams. 801-1015 grams. 



Males 349 651 954 



Females 299 621 912 



When the weight for the two sexes is here compared, it is seen that the 

 average for the female is the closer to the lower limit in each group. As by 

 hypothesis we are dealing with the least possible number of elements in either 

 sex, and as there is no reason to assume that this minimum number is materi- 

 ally different for the two sexes, the inference is plausible that in these cases 

 the difference in weight is in a large measure due to the difference in the size 

 of the constituent elements. If this holds for the lower limit of the series, it 

 is of course also probable that it holds throughout the entire series as well. 



As compared with the average brain, those of either sex forming the groups 

 heavier than the average owe their greater weight more often to an increase in 

 the size of the constituent elements than to an increase in their number. On 



1 Marchand : Nova Acta der Kaiserl. Carol. Deutsch. Akad. der Naturforscher, Halle, 1890. 



