Pelvis of the Chimpanzee
A, sacrum; B, fourth lumbar vertebra; C, coccyx; D, ilium or hip bone; E, femur or thigh bone.
Some writers have put great stress upon the difference in the structure of this bone, and have pointed out as impossible a common origin for man and ape; but one fact remains to be explained, and that is, that while these appear to be fixed and constant characteristics of man and ape there are many exceptions known in human anatomy. In the splendid collection of human spinal columns in the museum of the Harvard Medical School are no less than eighteen specimens of the human sacrum having six united segments; and I have found in the collections of various museums a total of more than thirty others. These facts show that this characteristic is not confined to the ape. It is true that in some of these abnormal specimens there remain five lumbar vertebræ. This seems to indicate that this portion of the spinal column is the most susceptible to variation. I have never seen an instance, however, of variation in the sacrum of the chimpanzee. In this respect he appears to be, in his structural type, more constant than man.
One reason why this bone is so formed in the ape is this. At that point the greatest weight and strain are laid upon the spinal column, and the crouching habit of the animal has a tendency to depress the lowest lumbar vertebra between the points of the hip bones and thus arrest its lateral movement. Since the flexure of this part is lessened, the cartilage that lies between the two segments becomes rigid and then ossifies. The erect posture of man allows more play in the region of the loins, and hence this motion prevents the two bones from uniting.
Another bone that may be said to vary somewhat is the sternum, or breastbone. It is the thin, soft bone to which the ribs are joined in the front of the body. In the young of both man and ape it is a mere cartilage. This slowly ossifies as the animal matures. The process appears to begin at five different segments, the first nucleus appearing near the top. This bone never becomes quite perfect either in man or ape. It always remains somewhat porous, and even in advanced age the outline of the lower portion is not defined by a smooth, sharp line, but is irregular in contour and merges into the cartilages that unite the ribs to it.
In an adult human being this bone is usually found to be in two segments, while in the ape it varies. In some specimens it is the same as in man. In others it is sometimes found to be in three, four, or even five sections. But the sternum in each is regarded as one bone, and is developed from one continuous cartilage. The separate parts are not considered distinct bones. The reason, no doubt, that this bone remains in separate sections in the ape is due to the stooping habit of the animal, by which the part is constantly flexed and alternately straightened, and therefore discharges its function better than it otherwise could.
With these trifling exceptions the skeletons of man and ape may be truly said to be exact counterparts of each other, having the same number of bones, of the same general model, arranged in the same order, articulated in the same manner, and performing the same functions. In other words, the corresponding bone in each is the same in design and purpose. The frame of the ape is, as a rule, more massive in its proportions than that of man; but while this is true of certain kinds of apes, the reverse is true of others.
In man the sacrum is more curved in the plane of the hips than it is in the ape, while the bones of the digits in man are less curved. The arms of man are shorter than the legs, while in the ape the comparative length of these features is reversed. In the cranial types it is readily seen that the skull of man is more spherical and the face almost or quite vertical. The skull of the ape is elongated and the chin projects. Thus his face is at an angle from a vertical line. These facts deserve more notice than the mere mention.
Diagram No. 1