Reginald J. Gladstone 
113 
hemisphere vesicles, which appear to be occasioned by the growth of the brain 
being more rapid than the capsule which confines it. At a still later stage 
following a more rapid enlargement of the skull many of the early fissures 
disappear, but they are afterwards replaced by the permanent fissures or sulci of 
the adult brain. The appearance and disappearance of these fissures seem to 
indicate that there are two counteracting forces concerned in the growth of the 
brain, namely an expanding force due to increase in brain matter, and cerebro- 
spinal fluid, and a passive restraining force exerted by the skull. 
In anencephalic monsters the brain is imperfectly developed ; the hemisphere 
vesicles appear at an early stage of development to have ruptured and collapsed, 
and the cerebro-spinal fluid to have escaped through a triangular aperture, which 
may often be recognized at the time of birth in the position of the anterior 
fontanelle. The cei'ebro-spinal fluid thus passes into the cavity of the amnion, and 
the amount of fluid within this sac becomes thereby much increased. 
In these cases the expanding force concerned in the growth of the skull is 
absent, and consequently the bones forming the vault of the skull, if they are 
developed at all, grow only to a sufficient size to cover over the base of the skull, 
and rudiments of the brain, whereas the bones of the face, and of the skeleton 
generally, grow to their normal size. 
In rickets on the other hand the bones of the skull are abnormally soft and 
yielding, and apparently do not afford the usual resistance to the expanding 
pressure from within, and as a result the cranial portion of the skull enlarges, and 
the w^ell-known square overhanging forehead of a typical rickety child is produced, 
while the face remains of the normal size and owing to the enlargement of the 
head above, appears unusually small. 
In the course of normal growth the two forces continue to counteract one 
another until the brain lias attained its maximum size. Later a progressive 
diminution in the weight and probably also of the size of the brain takes place, 
and the sutures of the skull afterwards, gradually consolidate. 
Not only is there the difference mentioned above in the general thickness 
of the skull at different ages, but there is also a considerable difference in the 
frontal region, due to the development after puberty of the frontal air sinuses, 
which tend to increase in size with the advance of age, and which are usually 
considerably larger in the male than in the female. Their positiun and the size 
which they may attain are well seen in Fig. 4, D, and Plate II., Figs. B and C. 
It is obvious, therefore, that in any attempt to calculate the weight of the brain 
age must be taken into account, since it affects not only the brain itself, but 
also the thickness of the scalp and of the skull. It may be noted, however, that 
after middle age the variations attributable to age are so far as the thickness of 
the scalp and the weight of the brain are concerned, in the same direction, namely 
diminution, so that the proportion between the outside measurements of the head 
and the brain- weight is not very materially altered. The diminution in brain- 
Biometrika iv 15 
