The calvaria possesses in its centre a dense curved rib, which extends through the roof from the anterior to the posterior part of the base, but which is more evident at its extre mities than in its middle, where it is generally marked by a groove for the longitudinal sinus. The frontal spine commences it, and its ter mination is the superior limb of the internal crucial spine ; the intermediate portion (where it is masked)- is the sagittal suture. On each side, and from before backwards, we notice in succession the frontal depression ; the coro na' suture; the parietal depression, and several arterial sulci running towards it from below ; part of the lambdoidal suture ; and, lastly, the cerebral fossa of the occipital bone. On each side of the sagittal suture are the foss Pacchioni, and, near its back part, the foramen parietale.
A comparison of the external and internal surfaces of the cranium establishes the fact that there is a general correspondence of the two as far as regards those parts which are in contact with the periphery of the brain. But, between the several divisions of that organ, there are developed on the inside of the skull very large ribs and processes which destroy the particular correspondence of the two surfaces.
Nevertheless, this does not impair our ability to deduce the internal capacity of the cranium from an examination of its exterior; since the diplue between the two plates, in the spaces intermediate to these ribs, seldom varies more than one or two lines in its thickness.
lti a skull of ordinary capacity, the length, measuring from the frontal spine to the longi tudinal sulcus, is five inches and a half; its width, between the bases of the petrous pro cesses of the temporal bones, four inches and a half; between the parietal fossw, five inches; and between the extremities of the alze mi nores, three inches and three quarters : its depth, from the foramen magnum, four inches and a half, from the ephippium three inches and a quarter; and, from the front of the olivary process, two inches and three quarters. But observation proves to us that there is little dependence to be placed on these measure ments ; scarcely any two skulls agree in their diameters, for where one exceeds in a given direction, it may fall short in some other. To this conclusion we shall be led by the ex amination of skulls, not only of members of the same community but even of persons con nected by the closest ties of consanguinity. While, hovvever, there is any doubt about the matter, it is not to mixed communities we should have recourse in our search for facts ; but rather to the well-authenticated skulls of such tribes as inhabit parts of the globe re mote from each other, and whose manners and customs have, to the best of our belief, re mained stationary from time immemorial; for by this procedure we shall avoid the confusion arising from a mixture of different races of men whose respective dispositions have been modified by intermarriage.
The skulls of a North American Indian and a Hindoo will be good examples to shew how the diameters will vary. By making a longi tudinal section of each, we shall find, by ap plying a line between a spot about five-eighths of an inch above the root of the nose, and another about three-eighths of an inch above the superior angle of the occipital bone, that there is considerably more space above the line in the Hindoo than there is in the American Indian, while the distance to the foramen magnum is much greater in the latter than in the former. Again, if we make the usual ho
rizontal section, it will be manifest that in breadth the Indian will exceed the Hindoo by nearly, and, sometimes, more than an inch, although the latter has the advantage in lencrth.
In the Negro, which, in length, is equal to the Hindoo, the space above the line in a vertical section is not absolutely, much less relatively, so great towards the frontal bone as in the shorter skull of the Indian ; while towards the posterior part of the parietals it is much greater, and in its breadth it falls but little short of it.
These three aboriginal types will suffice to shew the endless varieties which must prevail in mixed communities, and to satisfy us that the forms of skulls are as numerous as the diversified modifications of character with which the Creator has endowed the human race.
Several naturalists have sought to establish an analogy between the cranium and the ver tebra., and have imagined that they had dis covered in the one a type of the other; in other words, that the cranium is neither more nor less than a gigantic vertebra which has been submitted to some necessary modifications.
In this sense the ephippium and basilar por tion of the occipital bone represent the body of a vertebra; the foramen magnum, the ver tebral foramen ; the longitudinal spine of the occipital bone, the spinous process ; the ex panded portion of the bone as far as the mas toid portion of the temporals, the vertebral plates ; the mastoid processes themselves, the transverse processes; the eminence above the anterior condyloid foramina and the condyles themselves, the superior and inferior oblique processes; and the notch behind the condyles and the jugular notch, the notches which form the conjugal foramina.* Others again regard the cranium as com posed of several vertebrze more or less com plete, which are so associated as to meet the exigencies of the highly developed summit of the medulla spinalis. The resemblance, how ever, of many of the parts to a vertebra is so imperfect as to admit of the greatest license, as respects both the fixing of the number and the apportioning of the parts which severally belong to them. The alteration of position, too, to which they are necessarily subject to enable themsto accord with the change in direc tion which the nervous matter sustains, casts much confusion on the subject, and prevents the mind from recognizing, at once, a similarity which would be more apparent if they con tinued to be superimposed on each other as they are in the spine instead of being arranged at right angles with it.f The occipital bone certainly offers no dif ficulty to the detection of an analogy between it and a vertebra; and we readily discern in it a body ; a foramen ; two transverse, four arti cular and one spinous process; and four notches. These have already been pointed out, and it is sufficient here to observe, that, in this bone apart from the others, the basilar process alone will represent the body, and the lateral processes will be the type of the trans verse processes of the vertebra.