Page:Popular Science Monthly Volume 25.djvu/744

726 fibrous tissue. In many of the lower animals it is the normal condition. It is seen in nearly all the carnivora, except the plantigrades (though it has been found in the cave-bear); it is also seen in monkeys, lemurs, and sloths. In these it is generally completed by bone, though in some by bone and ligament as in man. In the animals above mentioned it serves the purpose of protecting the great nerve and vessel of the fore-limb from pressure during flexion, and it also affords a more direct course by which these structures can supply the parts below (see Fig. 4). In man when this arrangement occurs, owing to the altered position of the limb, the nerve and blood-vessel are actually dragged out of their course to pass through this opening; so in him it serves no useful purpose. This variation is, as was first pointed out by Professor Struthers, well known to affect certain families. The only reasonable explanation of the occurrence of this structure appears to be that of reversion to the type of some mammalian ancestor in which this part was functional, or in other words served a definite purpose (Struthers).

—The third trochanter of the thigh-bone occurs about as frequently as the supra-condyloid process. On the Tipper part of the thigh-bone there are two prominences called the greater and less trochanter; a third prominence (trochanter tertius) sometimes occurs; it is situated a little below the great prominence, and gives attachment to the large muscle of the buttock (glutceus maximus). According to Fürst, in forty skeletons of Swedes examined by him in the Caroline Institute in Stockholm, fifteen possessed this process, and, in six skeletons of Laplanders, four had a third trochanter. I have seen it in only about one per cent of the skeletons I have examined. In many of the lower animals this process is enormously developed; it is very prominent in the horse and rhinoceros, and in many others it exists in a slighter degree.

One more example from the osseous system, and I shall pass to the softer structures. In the human wrist are eight small bones called carpals, and arranged in two rows; occasionally between the two rows we have a ninth bone called the os centrale. This os centrale is always present in the higher apes and some of the rodents. We also find that in every human fœtus at an early period a rudiment of this bone exists, but it has entirely disappeared by the fourth month of fetal life.

—Every naturalist now admits that the various stages of development of an animal, as well as its specialized parts, are often found to correspond with permanent conditions of animals lower in the scale. A good illustration of this is seen in the development of the human heart and blood-vessels. In the early stages of development we have a heart with a single cavity, connected with a vessel at each end as in ascidians; later on the blood-vessels consist of a series of arches which go to the gills or branchial clefts as in fishes and amphibia, while the heart consists of two chambers separated by