OCR Text |
Show 150 "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 ques~ tion, 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 ·between a Rodent and a Man (Fig. 2g), be examined, it will be found that in the higher crania the basi-cranial 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. 30), 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 goo, is as much as 150° or 160°; the cranio-facial angle may be goo 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, accord- 151 ing 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 questi<?n in the affirmative. FIG. 30.-Sections of orthognatl}ous (light contour) and prognathous (dark contolll') skulls, one-third of the natural size. a b, Basicranial axis ; b c, b' c', plane of the occipital foramen ; d d', hinder end of the palatine bone ; e e', front ond of tho upper jaw; 1'T', insertion of the tentorium. |