Man's Place in Nature, and Other Essays

by Thomas Henry Huxley

But if a series of sections of mammalian skulls, intermediate between a Rodent and a Man ([Fig. 28]), be examined, it will be found that in the higher crania the basicranial axis becomes shorter relatively to the cerebral length; that the “olfactory angle” and “occipital angle” become more obtuse; and that the “cranio-facial angle” becomes more acute by the bending down, as it were, of the facial axis upon the cranial axis. At the same time, the roof of the cranium becomes more and more arched, to allow of the increasing height of the cerebral hemispheres, which is eminently characteristic of man, as well as of that backward extension, beyond the cerebellum, which reaches its maximum in the South American Monkeys. So that, at last, in the human skull ([Fig. 29]), the cerebral length is between twice and thrice as great as the length of the basicranial axis; the olfactory plane is 20° or 30° on the under side of that axis; the occipital angle, instead of being less than 90°, is as much as 150° or 160°; the cranio-facial angle may be 90° or less, and the vertical height of the skull may have a large proportion to its length.

It will be obvious, from an inspection of the diagrams, that the basicranial axis is, in the ascending series of Mammalia, a relatively fixed line, on which the bones of the sides and roof of the cranial cavity, and of the face, may be said to revolve downwards and forwards or backwards, according to their position. The arc described by any one bone or plane, however, is not by any means always in proportion to the arc described by another.

Now comes the important question, can we discern, between the lowest and the highest forms of the human cranium anything answering, in however slight a degree, to this revolution of the side and roof bones of the skull upon the basicranial axis observed upon so great a scale in the mammalian series? Numerous observations lead me to believe that we must answer this question in the affirmative.

Fig. 28.—Longitudinal and vertical sections of the skulls of a Beaver (Castor Canadensis), a Lemur (L. Catta), and a Baboon (Cynocephalus Papio), a b, the basicranial axis; b c, the occipital plane; i T, the tentorial plane; a d, the olfactory plane; f e, the basifacial axis; c b a, occipital angle; T i a, tentorial angle; d a b, olfactory angle; e f b, cranio-facial angle; g h, extreme length of the cavity which lodges the cerebral hemispheres or “cerebral length.” The length of the basicranial axis as to this length, or, in other words, the proportional length of the line g h to that of a b taken as 100, in the three skulls, is as follows:—Beaver 70 to 100; Lemur 119 to 100; Baboon 144 to 100. In an adult male Gorilla the cerebral length is as 170 to the basicranial axis taken as 100, in the Negro ([Fig. 29]) as 236 to 100. In the Constantinople skull ([Fig. 29]) as 266 to 100. The cranial difference between the highest Ape’s skull and the lowest Man’s is therefore very strikingly brought out by these measurements.
In the diagram of the Baboon’s skull the dotted lines d¹d², &c., give the angles of the Lemur’s and Beaver’s skull, as laid down upon the basicranial axis of the Baboon. The line a b has the same length in each diagram.

The diagrams in [Figure 29] are reduced from very carefully made diagrams of sections of four skulls, two round and orthognathous, two long and prognathous, taken longitudinally and vertically, through the middle. The sectional diagrams have then been superimposed, in such a manner, that the basal axes of the skulls coincide by their anterior ends, and in their direction. The deviations of the rest of the contours (which represent the interior of the skulls only) show the differences of the skulls from one another, when these axes are regarded as relatively fixed lines.

The dark contours are those of an Australian and of a Negro skull: the light contours are those of a Tartar skull, in the Museum of the Royal College of Surgeons; and of a well developed round skull from a cemetery in Constantinople, of uncertain race, in my own possession.

It appears, at once, from these views, that the prognathous skulls, so far as their jaws are concerned, do really differ from the orthognathous in much the same way as, though to a far less degree than, the skulls of the lower mammals differ from those of Man. Furthermore, the plane of the occipital foramen (b c) forms a somewhat smaller angle with the axis in these particular prognathous skulls than in the orthognathous; and the like may be slightly true of the perforated plate of the ethmoid—though this point is not so clear. But it is singular to remark that, in another respect, the prognathous skulls are less ape-like than the orthognathous, the cerebral cavity projecting decidedly more beyond the anterior end of the axis in the prognathous, than in the orthognathous, skulls.

It will be observed that these diagrams reveal an immense range of variation in the capacity and relative proportion to the cranial axis, of the different regions of the cavity which contains the brain, in the different skulls. Nor is the difference in the extent to which the cerebral overlaps the cerebellar cavity less singular. A round skull ([Fig. 29], Const.) may have a greater posterior cerebral projection than a long one ([Fig. 29], Negro).