Page:Evidence as to Man's Place in Nature.djvu/179

Rh, and presplienoid, 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.'

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°.

But if a series of sections of mammalian skulls, intermediate beween a Rodent and a Man (Fig. 29), 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,