RELATIVE WEIGHT AND VOLUME OF COMPONENT PARTS OF FETAL BRAIN. 57 



tenth to one-third less than the original volume) , the interest attached to the ques- 

 tion of the volume of the fetal brain perhaps warrants my including the data given 

 in table 2, which were obtained on the basis of the volume of the models. From 

 these same figures one can readily calculate the volume of any individual part of 

 the brain by multiplying the total volume by the percentage of the total brain 

 formed by that particular part (table 3) . 



SUMMARY. 



A study of the preceding data shows the rapid growth of the brain as a whole 

 and the relatively enormous rate of growth of some of its component parts as com- 

 pared with others; for, while all parts of the brain consistently increase in size, they 

 do not all grow at the same rate. The telencephalon, for example, shows a relatively 

 rapid increase throughout the entire series, greatly surpassing all other parts; the 

 cerebellum shows a similar, though less marked increase during the latter half of 

 gestation; while all of the other parts show a relative decrease in weight- values. 



Starting with an embryo of 4 3/2 weeks, 4.3 mm. long, with an actual total brain 

 volume of approximately 0.003 c. c., and in which it is possible to definitely outline 

 little more than the three primary brain vesicles common to all vertebrates, one can 

 follow the rapid growth and development until birth, when this complex and highly 

 specialized organ attains a total actual volume of 490 c. c., a gain in less than 30 

 weeks of over 160,000 times the initial volume. 



An analysis of the values for the different parts shows a steady upward curve 

 for the prosencephalon, from 31.3 per cent in the early embryo to 92.38 per cent at 

 term, a gain of nearly three times its initial bulk, despite the fact that all of its 

 component parts, except the telencephalon, have shrunken considerably in weight- 

 values. The percentage of the total brain-weight formed at different stages by the 

 five chief subdivisions of the brain is shown graphically in chart 1. 



The most striking feature in this growth curve, as well as the most significant 

 one, is to be found in the enormous proportional and actual increase in the size of the 

 telencephalon, which gives the amount of cortical expanse necessary to provide for 

 the control of all subsidiary parts of the nervous system, as well as being the seat 

 of the higher psychic functions. Beginning with an initial weight of 7 per cent at 

 4> weeks, the telencephalon increases rapidly up to 13.K weeks, when it constitutes 

 80.56 per cent of the entire encephalon. Then follows a period of more gradual 

 growth up to the twenty-sixth week, when the telencephalon attains its maximum 

 relative weight (89.62 per cent) which it maintains until term. Assuming that the 

 specific gravity of the brain tissue is very little over 1.0, the actual increase in weight 

 is from approximately 0.0002 gram (as calculated from volume) at 4>^ weeks to 

 493.8 grams at birth. 



Another noteworthy point in the growth rate of the cerebral hemisphere is that 

 it attains its maximum relative weight before any folding occurs to increase the 

 extent of its periperhal surface, showing that in the later weeks its development is 

 one of complexity rather than of bulk, a fact which would appear to argue that these 

 later changes consist in perfecting the development of already existing structural 

 units, rather than in the acquisition of new ones. 



