Lectures and Essays

by Thomas Henry Huxley

(FIGURE 26.—Side and front views of the round and orthognathous skull of a Calmuck, after Von Baer. One-third the natural size.)

I have arrived at the conviction that no comparison of crania is worth very much, that is not founded upon the establishment of a relatively fixed base line, to which the measurements, in all cases, must be referred. Nor do I think it is a very difficult matter to decide what that base line should be. The parts of the skull, like those of the rest of the animal framework, are developed in succession the base of the skull is formed before its sides and roof; it is converted into cartilage earlier and more completely than the sides and roof: and the cartilaginous base ossifies, and becomes soldered into one piece long before the roof. I conceive then that the base of the skull may be demonstrated developmentally to be its relatively fixed part, the roof and sides being relatively moveable.

(FIGURE 27.—Oblong and prognathous skull of a Negro; side and front views. One-third of the natural size.)

The same truth is exemplified by the study of the modifications which the skull undergoes in ascending from the lower animals up to man.

(FIGURE 28.—Beaver, Lemur and Baboon. Longitudinal and vertical sections of the skulls of a Beaver ('Castor Canadensis'), a Lemur ('L. Catia'), 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 (Figure 29) as 236 to 100. In the Constantinople skull (Figure 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 'd1 d2', etc., 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.)

In such a mammal as a Beaver (Figure 28), a line ('a b'.) drawn through the bones, termed basioccipital, basisphenoid, and presphenoid, is very long in proportion to the extreme length of the cavity which contains the cerebral hemispheres ('g h'.). The plane of the occipital foramen ('b c'.) forms a slightly acute angle with this 'basicranial axis,' while the plane of the tentorium ('i T'.) is inclined at rather more than 90 degrees to the 'basicranial axis'; and so is the plane of the perforated plate ('a d'.), by which the filaments of the olfactory nerve leave the skull. Again, a line drawn through the axis of the face, between the bones called ethmoid and vomer—the "basifacial axis" ('f e'.) forms an exceedingly obtuse angle, where, when produced, it cuts the 'basicranial axis.'

If the angle made by the line 'b c'. with 'a b'., be called the 'occipital angle,' and the angle made by the line 'a d'. with 'a b'. be termed the 'olfactory angle,' and that made by 'i T'. with 'a b'. the 'tentorial angle,' then all these, in the mammal in question, are nearly right angles, varying between 80 degrees and 110 degrees. The angle 'e f b'., or that made by the cranial with the facial axis, and which may be termed the 'cranio-facial angle,' is extremely obtuse, amounting, in the case of the Beaver, to at least 150 degrees.

But if a series of sections of mammalian skulls, intermediate between a Rodent and a Man (Figure 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 America Monkeys. So that, at last, in the human skull (Figure 29), the cerebral length is between twice and thrice as great as the length of the basicranial axis; the olfactory plane is 20 degrees or 30 degrees on the 'under' side of that axis; the occipital angle, instead of being less than 90 degrees, is as much as 150 degrees or 160 degrees; the cranio-facial angle may be 90 degrees 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.