OCR Text |
Show 148 modifications of that devised by Peter Camper, in order to attain what he called the 'facial angle.' 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. ]'or 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 developll!entally 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. 29), 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 tent<;>rium (i. T.) is inclined at rather more than 90° to the ' basicranial axis' ; and so is the plane of the perfo- 149 rated 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 Bea.ver. Lemur. Baboon. FIG. 29:-Longitudinal and vertical sections of the skulls of a Beaver (Castor Oanad~ns~s)! a L~mur (L. Oatt'!'~' and a Baboon (Cynocephalus Papio), a b, the bas1cramal ax1~; b c. th~ oc~Ipital_plane; iT, the tentorial plane; ad, tho olfactory plane; j e, the bas1facml ax1s; c b a, occipital angle; T i a, tentorial ang~e; d r; b, olfactory angle ; ef b, cranio-facial angle; g h, extreme length of the cavttywh~ch l~dges _the cereb;al hemisphe.res or' cerebral length.' The length of the b_asiCramal axis as to this length, or, mother words, the proportional length of the hne 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. 30) ~s 2~6 to 100. In the Constantinople skull (fig. 30) ;s 266 to 100. The cramal difference between the highest Ape's skull and the lowest Man's is therefore ':ery strikingly brought out by these measurements. In the d1agram 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 a..'l:is of the Baboon. The hne a u bas the same length in each diagram. |