RELATION BETWEEN WEIGHT OF BRAIN AND TOTAL WEIGHT
(According to Massini)
| Age | Brain | Body | Age | Brain | Body |
|---|---|---|---|---|---|
| At birth | 1 | 8 | 2 years | 1 | 15 |
| First month | 1 | 9 | 3 years | 1 | 14 |
| From first to third month | 1 | 9 | |||
| to sixth month | 1 | 10 | |||
| one year | 1 | 12 | 25 years | 1 | 40 |
In other words, the body grows more rapidly than the brain, and consequently, than the head: a fact which results in the different proportions already noted between head and body.
The rhythm of brain growth considered by itself has been set forth in a most noteworthy and accurate fashion by Boyd, based on the study of about two thousand cases; from the figures given by Boyd, I have calculated the amount of increase from period to period, as well as from year to year, the whole result being set forth in the following table:
RHYTHM OF GROWTH OF BRAIN
(Males: According to Boyd)
| Age | Weight in grams | Difference for each period | Difference for each year | Relative epoch | Proportion to maximum reduced to 100 |
|---|---|---|---|---|---|
| At birth | 331 | — | — | — | 24.2 |
| From birth to 3 months | 493 | +162 | — | — | 36.0 |
| From 3 to 6 months | 603 | +110 | — | — | 44.1 |
| From 6 months to 1 year | 777 | +174 | +446 | 1st year | 56.8 |
| From 1 to 2 years | 942 | +165 | +165 | 2d year | 69.0 |
| From 2 to 4 years | 1,097 | +155 | +77 | 2d- 4th | 80.4 |
| From 4 to 7 years | 1,140 | +43 | +14 | 4th- 7th | 83.4 |
| From 7 to 14 years | 1,302 | +162 | +23 | 7th-14th | 95.3 |
| From 14 to 20 years | 1,374 | +72 | +12 | 14th-20th | 100.5 |
| From 20 to 30 years | 1,357 | — | — | — | 99.3 |
| From 30 to 40 years | 1,366 | +9 | +0.9 | 30th-40th | 99.3 |
| From 40 to 50 years | 1,352 | -14 | -1.4 | 40th-50th | 98.9 |
| From 50 to 60 years | 1,343 | -9 | -0.9 | 50th-60th | 98.3 |
| From 60 to 70 years | 1,315 | -28 | -2.8 | 60th-70th | 96.9 |
| From 70 to 80 years | 1,289 | -26 | -2.6 | 70th-80th | 95.3 |
| From 80 to 90 years | 1,284 | -5 | -0.5 | 80th-90th | 94.2 |
In the above table, the first column of figures gives the mean average weight of the brain, obtained by direct observation of individual subjects; while from all the others the rhythm of cerebral growth and involution throughout the successive periods of life may be computed.
We see that the maximum growth takes place in the first years of life, the intensity is greater in the first year than in the second, and greater in the first three months than in those that follow. Already at the end of the first year the brain has surpassed one-half of the maximum weight which the individual is destined to attain in adult life (last column: proportions computed on scale of 100). A notable rate of increase continues up to the age of four, after which it moderates, but receives a new impulse at about the fourteenth year (period of puberty); hence it appears that at this important epoch of life the brain not only shares the general rapid growth of the body, but that by the end of the fourteenth year the brain has already practically completed its development; in fact, assuming that 100 represents its complete development, the weight of the brain is already 95.3; and at thirty it will be only 99.3.
By studying the above table we can obtain a clear analysis of these phenomena.