Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/173

Rh SKELETON.] A P E 159 ossified, as in the Cebince and Mycetes. From Hylolates downwards there is a small cerebellar fossa on the inner surface of the petrosal. The Vertebral Column. The vertebral or spinal column in all apes consists of the same regions as it does in man, and, with the exception of the coccygeal or caudal portion, these regions bear more or less the same proportions one to another as they do in him. That beautiful sigmoid curvature which exists in the human spine is not found in any ape; but the nearest approximation to it is found, not in the Simiince, but in the Cynocephali. In the latisternal apes the spinous pro cesses are directed as in man, but in the rest their apices converge towards a single point in the back. The cervical region is relatively more elongated than in man in Hylobates, Cynocephahcs, and Ateles. The atlas is very man-like in Troglodytes and Simla, except that its transverse process is rather smaller and shorter relatively, and more curved dorsally. Except in Ateles and the Simiince, it is almost always perforated by the vertebral artery, instead of being merely grooved by it. In Cyno- ceplialus the atlas is exceedingly massive, and may, by very rare exception, develop a spinous process. The spines of the cervical vertebrae do not bifurcate beyond the second, and generally the spine of the seventh vertebra is the longest, at least &quot;with the exception of that of the axis. In the gorilla and orang, however, the spines of the third, fourth, tifth, and sixth vertebrae are exceedingly elongated and longer than that of the seventh. The cervical neural laminre are wider antero-posteriorly, the vertebral column being supposed horizontal, in Hylolates and Ateles than in other forms. Beneath the anterior articular process (prezygopophysis) a marked prominence the metapophysis may be developed, and is particularly conspicuous in Ateles. The costal plate of the sixth cervical is almost always larger than the homologous part in the other cervicals ; but it predominates least in the chimpanzee and Xyctipithecus. The length of each centrum, compared Avith its breadth, is already greater in Troglodytes and Simla than in man. In others the dimensions are about equal, except that in CynocepJialus, Ateles, and Mycetes the length slightly exceeds the breadth. The dorsal vertebrae vary in number from eleven, as sometimes in Cercopithecus and Macacus, to fourteen, as sometimes in llylobatcs, or even to fifteen, as in Nycti- pitliecus. In the Cebidce, above the Hapalince, the number rarely falls below thirteen. In the orang the number is, as in man, twelve; in Troglodytes it is thirteen, The pro portion of this region does not increase regularly with the number of its vertebra}. It attains its greatest relative length in Ateles, where it is over nine-twentieths of the total length of the spine without the tail ; while in Nycti- jnthecus it is relatively shorter thac in Troglodytes or Simia. The increase in transverse diameter of the bodies of the vertebra?, as we proceed from before backwards, is exceptionally great in Hylobates and CynocepJialus. The lumbar region contains from four to seven vertebras In the Simiince and Cebince there are four or five, though in Cebus the number is often six. Its relative length, is greatest in Nyctipithecus, Chrysothrix, the SemnopitJie- cince, and Cynoceplialus. It is least in the Simiince and Ateles. The number of dorsal and lumbar vertebrae, taken together, is seventeen in Troglodytes and Simia, eighteen (mostly) in Ateles and Hylobates, and nineteen in the rest. In most forms the dorsal and lumbar regions are about equal in length, but the lumbar region is tho shorter in the Simiince, and less than half the length of the dorsal in the gorilla. The lumbar spinous processes are vertical, or project backwards in the Simiince and in Ateles ; in the other forms they project strongly forwards, especially in the lower Cebidce. The lumbar transverse processes always project outwards, more or less at right angles to the axis of the spine, or else forwards. In the Simiince and Aides they are never inclined ventrally, but in some of the lower Simiadce they begin to be so ; in the lower Cebidce they are so very decidedly. Only in Troglo dytes and Simia are they inclined somewhat dorsally. In the last two genera the lumbar vertebrae are broader than their length ; in most other forms length is in excess. The length of the individual lumbar vertebra), compared with the dorsal (e.g., the longest lumbar with the ninth dorsal), is greatest in the lower Cebidce, rather less in the lower Simiadoe, and least in the Simiince. The sacrum attains its greatest absolute length in the gorilla, but it is relatively longer than in man in all the Simiince. Of all the inferior apes, Hylobates has the relatively longest sacrum. The number of vertebrae in- included in the sacrum varies more or less with age. In the apes below the Simiince there are generally only two or three sacral vertebrae. In Ateles, Hylobates, and BracJiy- urus, there may be four ; while in the Simiince there are always five, and sometimes six sacral vertebrae. In most apes the sacrum and lumbar vertebrae appear to lie in one slightly curved line. Troglodytes presents in this respect a great contrast to the human structure. In Simia the sacro- vertebral angle is rather more marked; but sometimes in CynocepJialus it is so much so as almost to rival that of man. The same may be said of the concavity of the anterior surface of the sacrum. It is most marked in the Cyno- cephali, and is more so in Simia than in Troglodytes. The sacrum gradually tapers posteriorly in Troglodytes and Simia, and more or less so in Hylobates. In the other Simiadce, however, the sacrum is very wide at its anterior end, and it then very rapidly contracts backwards, so that the transverse diameter of the third sacral vertebra is very much narrower than that of the first. In the Cebidce generally, and especially in Ateles, the contraction is not so marked. The caudal vertebrce are always more than four in number, except in the Simiince and in Macacus inuus. In Cyno- FIG. 17. Skeleton of Atclfs Sdzebuth. From De Blatnville. cephalus mormon (the mandrill), however, there are some times only five vertebrae. The short-tailed Macaci and Brachyuri have from about fifteen to seventeen caudal vertebrae, the shortness of the tail being occasioned rather by a diminution in the size of the component vertebrae than by a decrease in their number. In the other forms the number varies between twenty and thirty-three, the latter being the number attained in the genus Ateles. The