20 
Variation and Cor7^elation in Brain-Weight 
hospital population mentioned above, namely, " persons acutely ill." It is at once 
clear that this will include two sub-groups. First in number and importance will 
be the group of individuals suffering from diseases medically classified as "acute " 
in the strict sense of the term, for example, typhoid fever. Second will be the 
group of individuals acutely ill at the termination of a long-standing chronic 
disease. These individuals in most instances have been cared for at home as 
long as the disease remained in the chronic or sub-acute form, and only go into 
the hospital when it becomes acute. A good example of such cases is afforded by 
Bright's disease. There can be little doubt on a priori grounds that the mean 
age of the individuals will be lower in the first as compared with the second of 
these groups. This is shown to be the case by the actual statistics in the brain- 
weight series where " cause of death " is tabulated. In the lower age groups the 
causes of death returned are preponderantly " acute " in the strict sense. In the 
higher age groups we get a preponderance of the chronic affections. Now it is 
altogether likely that the acute illness which leads to death in a comparatively 
short time has much less effect on the weight of the brain than the chronic 
"wasting" sickness. So, then, the general result is that in the higher age 
classes where the brain-weight has decreased as the natural result of senescence 
there is a further artificial lowering on account of the preponderance of individuals 
who have been afflicted with " wasting " diseases. On the other hand the returns 
during early adult life give mean values for the brain-weight which are probably 
nearer the true normal value for the general population. 
In the present paper it was desired to consider only adult brain-weights and 
consequently only material which fell in age between 20 and 80 years was usually 
used. It is the general opinion of anatomists (cf. for example, Ziehen, loc. cit. 
p. 359, and Marchand, loc. cit. p. 404), that after the age period 15 — 20 there is 
very little increase of weight in the brain with advancing age. In other words 
the brain is considered to attain practically its complete growth in about the first 
20 years of life. According to Marchand {loc. cit. p. 402 et seq.) the maximum 
brain-weight remains practically constant till the individual is about 50 years old. 
After that age he believes that senile degeneration begins. Accordingly, in the 
discussion of the Marchand data, I have considered the period from 15 — 80 years 
to constitute the "adult period," so far as brain-weight is concerned. As a matter 
of fact, the result would have been but little diffei'ent had the period 20 — 80 
years been used in this case (as it was with all the other data), because there 
are only 36 males and 17 females falling in the age class 15 — 19, and these 
individuals are fairly evenly distributed among the brain-weight classes as will 
be seen by consulting Tables 17 and 18 of the Appendix. When the work was 
begun I had, of course, no means of knowing how it would turn out, so it was 
necessary to decide how to treat the material on the basis simply of a careful 
general inspection of the statistics and of the opinion of other workers on the 
subject. As one point which I wished especially to investigate was the change 
of brain-weight with age, it became a nice problem as to how to handle the 
