The next series of characters to be con sidered, are those by which man is adapted to the erect attitude. On examining his cra nium, we remark that the occipital condyles are so placed, that a perpendicular dropped from the centre of gravity of the head would nearly fall between them, so as to be within the base on which it rests upon the spinal column. The foramen magnum is not placed in the centre of the skull, but just behind it ; so that the greater specific gravity of the pos terior part of the head, which is entirely filled with solid matter, is compensated by the greater length of the anterior part, which contains many cavities. There is, indeed, a little over compensation, which gives a slight prepon derance to the front of the head, so that it drops forwards and downwards, when all the muscles are relaxed ; but the muscles which are attached to the occiput are larger and far more numerous than those in front of the condyles, so that they are evidently intended to counteract this disposition ; and we find, accordingly, that we can keep up the head for a whole day, with so slight and involuntary effort, that no fatigue is produced by it. More over, the plane of the foramen magnum, and the surfaces of the condyles, have a nearly horizontal direction when the head is upright ; and thus the weight of the skull is laid verti cally upon the top of the vertebral column. If these arrangements be compared with those which prevail in other Mammalia, it will be found that the foramen and condyles are placed in the latter much nearer the back of the head, and that their plane is more oblique. Thus, whilst the foramen magnum is situated, in Alan, just behind the centre of the base of the skull, it is found in the Chimpanzee and Orang to occupy the middle of the poste rior third ; and as we descend through the scale of Mammalia, we observe that it gra dually approaches the back of the skull, and at last comes nearly into the line of its longest diameter, as we see in the Horse. So the angle of inclination which the condyles make with the horizontal is very small in Man, but rises in the Orang to 37° ; whilst in the Horse their plane is vertical, making the angle 90°. If, therefore, the natural posture of man had been horizontal, the plane of his condyles would be brought, like that of the horse, into the vertical position ; and the head, instead of being nearly balanced upon the summit of the vertebral column, would hang at the end of the neck, so that its whole weight would have to be supported by some external and constantly-acting power. But for this there is neither in the skeleton, the ligamentous apparatus, nor the muscular sys tem of Man, any adequate provision ; so that in any other than the vertical position, his head, which is relatively heavier than that of most Mammalia, would be supported with more difficulty and effort than it is in any other animal.
The position of the face immediately be neath the anterior portion of the cranial cavity, so that its front is nearly in the same plane as the forehead, is peculiarly character istic of Man ; for the crania of the Chimpanzee and Orang, which approach most nearly to that of man, are entirely posterior to, not above, the face (see figs. 800-802.). It should be remarked that in the young ape, there is a much greater resemblance to man in this re spect, than there is in the adult ; for it is at the time of the second dentition, that the muzzle of the ape acquires its peculiar elonga tion and consequent projection in front of the forehead ; and the whole cast of the features is altered at the same time, so as to approxi mate much more closely to that of the lower Quadrumana than would have been thought likely from the inspection of the young ani mal only. This projection of the muzzle, taken in connection with the obliquity of the condyles, is another evidence of want of adaptation to the erect posture ; whilst the want of prominence in the face of Man shows that none but the erect position can be natural to him. For supposing that
with a head formed and situated as at pre sent, he were to move upon all fours, his face would be brought into a plane parallel with the ground ; so that as painful an effort would be required to examine with the eyes an object placed in front of the body, as is now necessary to keep the eyes fixed on the zenith ; the nose would then be almost inca pacitated for receiving any other odorous emanations, than those proceeding from the earth or from the body itself; and the mouth could not touch the ground, without bringing the forehead and chin also in contact with it. The obliquity of the condyles in the Qua drumana enables them without much difficulty to adapt the inclination of their heads either to the horizontal or to the erect posture; but the natural position, in the highest among them, is unquestionably one in which the spinal column is inclined, the body being par tially thrown forwards, so as to rest upon the anterior extremities ; and in this position, the face is directed forwards without any effort.
The vertebral column in Man, although not absolutely straight, has its curves so arranged, that when the body is in the erect posture, a vertical line from its summit would fall exactly on the centre of its base. It increases con siderably in size in the lumbar region, so as to be altogether somewhat pyramidal in its form. The lumbar portion, in the chim panzee and orang, is by no means of the same proportional strength, and contains but four vertebrm instead of five. The processes for the attachment of the dorso-spinal muscles to this part, are peculiarly large and strong in man ; and this arrangement is obviously adapted to overcome the tendency, which the weight of the trunk in front of the column would have to draw it forwards and down wards. On the other hand, the spinous pro cesses of the cervical and dorsal vertebrm, which in other Mammalia are large and strong, for the attachment of the ligaments and muscles that support the head, have compara tively little prominence in man, whose head is balanced upon the summit of the coltunn.
The base of the human vertebral column is placed on a sacrum of greater proportional breadth than that of any other animal ; this sacrum is fixed between two widely expanded ilia ; and the whole pelvis is thus peculiarly broad. In this manner the fe moral articulations are thrown very far apart, so as to give a wide basis of support ; and by the oblique direction of the axis of the pelvis, the weight of the body is trans mitted almost vertically from the top of the sacrum to the upper part of the thigh bones. The pelvis of the anthropoid apes is very differently constructed. That of the orang, for example, is much longer and narrower ; its alm extend upwards rather than outwards, so that the space between the lowest ribs and the crest of the iliac bones is much less than in man ; their surfaces are nearly parallel to that of the sacrum, which is itself longer and narrower ; and the axis of the pelvis is nearly parallel with that of the verte bral column. The position of the human femur, in which its head is most securely retained in its deep acetabulum, is that which it has when supporting the body in the erect attitude; in the chimpanzee and orang, its analogous position is at an oblique angle to the long axis of the pelvis, so that the body leans forwards in front of it ; in many of the four-footed Mammalia, as the elephant, it makes a right angle with the vertebral column ; and in several others, as the horse, ox, &c., the angle which it makes with the axis of the pelvis and vertebral column is acute. In this respect, then, the skeleton of man presents an adaptation to the erect pos ture, which is exhibited by that of no other mammal.