Fig. 26.—Side and front views of the round and orthognathous skull of a Calmuck after Von Baer. One-third the natural size.
But a little consideration will show that any “facial angle” that has been devised, can be competent to express the structural modifications involved in prognathism and orthognathism, only in a rough and general sort of way. For the lines, the intersection of which forms the facial angle, are drawn through points of the skull, the position of each of which is modified by a number of circumstances, so that the angle obtained is a complex resultant of all these circumstances, and is not the expression of any one definite organic relation of the parts of the skull.
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.
The same truth is exemplified by the study of the modifications which the skull undergoes in ascending from the lower animals up to man.
In such a mammal as a Beaver ([Fig. 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° 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.”
Fig. 27.—Oblong and prognathous skull of a Negro; side and front views. One-third of the natural size.
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° and 110°. 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°.