Page:Popular Science Monthly Volume 25.djvu/743

Rh there are more, and, when this occurs, the extra ribs are carried by the neck (cervical) or loin (lumbar) vertebræ. I have specimens in my collection of both varieties, cervical and lumbar (see Fig. 2, C), These supernumerary ribs do not occur very frequently; still, every anatomist has observed them. Their occurrence becomes more intelligible when we know that in crocodiles, birds, and the three-toed sloth, neck or cervical ribs exist normally; that in crocodiles, alligators, and some other animals, loin or lumbar ribs are never absent; and that in man traces of them exist in the muscles of the abdomen. In the human embryo, in an early stage, a rib is always seen connected with the seventh neck-vertebra, but before the fifth year of life it becomes blended with the ordinary transverse process (Fig. 2, T); occasionally, however, this rudiment goes on developing, till it becomes a more or less perfect cervical rib (see Fig. 2, C).

—It is not uncommon to find, in the humerus or arm-bone of man, a hook-like process on the inner side of the lower end, having a downward direction; this, with a band of ligament which connects its tip with the humerus lower down, forms a foramen or opening through which pass the great artery and nerve of the arm (see Fig. 3, A, B). This foramen is found in about three per



cent of recent skeletons, but much more commonly in the skeletons of ancient races. In very many bodies a trace of this foramen is seen, represented by a very small bony prominence, or only by a band of