The weight of the brain proper (cerebrum) has, at birth, a minimum figure of from 305 to 320 grams, a maximtun figure of from 310 to 345 grams. The extra-uterine gain in weight may, in a general way, be placed at from 830 to 840 grams, reaching in certain instances how ever, as high as 910 grams. At ail ages, the average weight of the male brain is greater than that of the female. It is the weight of the cere brum that heredity more particularly influences.
Next to weight, the dimensions of the child's brain give us an insight into the pronounced metamorphosis which takes place after birth. From the original nteasurements of 9 cm. in the fronto-occipital pole-length, 7 cm. in width and 5 cm. in height, does the brain acquire, often enough even before the time of puberty, the ultimate dimensions of 16-1S, 13-15, S-9 cm. respectively.
A feature which is very distinctive of the cerebrum of children, and more particularly of that of the newborn, is the relatively poor development of certain of its parts. Thus, for instance, the frontal lobe, especially in the prematurely-born, is decidedly inconspicuous. The insula of Red, though its external markings may be appreciable, is not as distinctly outlined in the first months of life as it is later.
In conneetion also with the fissures of the brain, are peculiarities to be noted in the new and prematurely-born. The primary fissures are, in proportion to the general development of the organ, relatively deeper during the first quarter of life than later. It is probable that as long as the phase of rapid growth determined by the investment of nerve-fibres with myelin sheaths lasts, further changes take place in connection with tbe shallower fissures of the convexity, such as con fluence of superficial sulci (which, in the parieto-oceipital region, occur with espeeial frequency during the first weeks of life), or deepening of fissures owing in the latter ease, to the progressively increasing promi nence of certain convolutions.
The cerebellum of the child also presents certain points of interest. From about IS to 21 grams at birth, its weight increases in the mune of development to 120 or 130, and even to 135 and 150 grains. The first two-thirds of this increase in the weight of the cerebellum takes place much sooner than the corresponding increase in the weight of tile cerebrum or of the brain generally, being usually manifest any where from the sixth month to the end of the second year; whereas its ultimate weight, eorresponcling to the very slow progress of its fur ther growth, rarely obtains before the end of the second decade. The average weight of the female cerebellum is invariably less than that of the male. This difference between the SCX.C51 increases front about three grams at birth, to about 15 or 20 grains in adult life. Inde
pendently of total body weight or general development, and in subjects of the same age, by no means trivial variations may be. observed in the size of the cerebellum, ranging all the way from 10 grams during the first months of life to 30 grams and more later. As early as the eleventh or twelfth year, the weight of the cerebellum may equal or exceed that of the fully mattured organ; which circumstance, however, does not rily imply a corresponding increase in the total weight of the encephalon.
The brain stent (medulla, pons and quathigeminal region) increases in weight from about 5.5 grams at birth to about 27 or 2S grams, in direct proportion to the growth of the brain in general. It is slightly larger in males than in females.
From the foregoing facts, it follows that the brain as a whole in the course of development quadruples its original weight at birth; the cerebrum likewise, almost as much; whereas the brain stein increases to five, and the cerebellum to seven times its miginal weight. Thus, is the (h•velopment of the cerebellum not only more rapid, as has al ready been shown, but, in proportion to its original weight, also much mom considerable, than that of the other parts of the brain. For this reason, the relative proportion between the size of the various seg ments of the brain is subject to constant variatio»; the eerebrum sinks from almost 93 per cent. of the total brain weight (at birth) to 87.5 per cent.; the brain stem rises from 1.6 per cent. to 2 per eent. and the cerebellum from 5.5 per eent. to almost 11 per cent. uf the total weight, As regards the spinal cord, considered as beginning at the inferior limit of the pyramidal deeussation (and deprived of its nerve roots and dura mater), its average weight in the newborn varies between 3.0 and 3.1 grams. and between 27 and 28 grams in adults. Its weight increase,: rapidly at the beginning of extra-uterine life, being doubled by the fifth month, trebled at the end of the first year and quadrupled at the beginning of the third year. Its average weight is practically always greater in males than in females. The relative pro portion between the weight of the spinal cord and that of the brain also differs in children from that adults; being in the latter about as 1: 50. In the newborn this proportion equals 1:110 at the very most; it becomes 1: SO in the third vear and reaches 1:67 by the end of the sixth year. Thus, with advancing age, the weight of the spinal cord becomes ever greater, relatively, than that of the brain; whereby it necessarily remains relatively less in males than in females of the same age. In children of the same age and sex, a heavy brain gener ally implies a proportionately heavy spinal cord.