168 
DR. J. CLELAND ON THE VARIATIONS OF THE HUMAN SKULL. 
the writer the explanation seems rather to be found in the consideration that the process 
of growth in the frontal and parietal bones and occipital squama consists normally of 
two parts, — the addition of new bone to the margins, extending the planes in which those 
margins lie, and the change of shape of each bone, by bending and unbending, from the 
form of a cone with truncated apex at the centre of ossification to a more uniformly curved 
but, on the whole, flatter shape. In the artificially compressed skulls the frontal and occi- 
pital bones are flattened out, while the parietals are forcibly bent ; there is no original 
tendency of the flattened central parts of the frontal and occipital bones to become ele- 
vated, and the natural tendency of the parietals to become flatter is insufficient to undo 
the enormous bending to which they have been subjected ; while, in addition, the direc- 
tions of the planes of marginal growth are entirely changed. 
Y. COMPARISON OF THE HUMAN SKULL WITH THE SKULLS OF YARIOUS ANIMALS. 
On comparing the human skull with that of the Chimpanzee or Orang, the most inter- 
esting points noticeable in the latter, in connexion with the facts brought forward in this 
paper, are the smallness of the cranial curvature, and the length of the base as compared 
with that of the arch. There is also a complete absence of balance of the head on the 
vertebral column, which is in harmony with the fact that no Ape is capable of supporting 
itself by balance on its hind limbs, but requires persistent muscular action to prevent it 
from falling. 
To compare the human cranium properly with crania of animals, the cranial cavity 
must be regarded as a dilated and curved continuation of the spinal canal. The advance 
in the form of the cranial cavity of Man, as compared with that of the Chimpanzee, 
consists in increased dilatation both in height and breadth and in increased curvature, 
whereby not only is the vault expanded, but the bones of the base are crowded together, 
the postsphenoid and presphenoid being fused, the ethmoid depressed, and the vomer 
pushed back in the way more fully described by the writer on a former occasion*. It is 
curious to note that while the dilatation of the cavity by height is greater in the Orang 
than in the Chimpanzee, the curvature is much greatest in the Chimpanzee"(Plate XXI.). 
It is impossible, however, in examining the curvature in the lower animals, to use any 
longer the means of estimating it which have served us hitherto ; for as we pass to lower 
forms we find the frontal bone coming further down on the face and the optic foramen 
varying in direction, so that the line joining the optic foramen and fronto-nasal suture 
no longer indicates any thing with regard to the cranial cavity. Another difficulty, when 
it is sought to compare the forms of brains, is that the roofs of the orbits, which form 
the floor of the anterior lobes of the brain, project to a variable and often considerable 
extent above the level of the cribriform plate. The writer has therefore, with the view 
of comparing the curvature of the brain in Man and Animals, availed himself of casts of 
the interiors of the crania of Dean Swift, an Australian, and a Gorilla, prepared under 
the direction of Professor Weight of Dublin, together with the casts of the interiors of 
* Philosophical Transactions, 1862, p. 296. 
