CHAPTER V
THE LIMBS
The study of the limbs is of great importance, because, although it is the special province of the bust to contain the organs of vegetative life, it is the limbs which render it useful. In fact, it is the lower limbs which control our locomotion and the upper limbs which execute the labour of mankind.
One characteristic of man, equally with that of standing in an erect position, supported only on the lower limbs, is the independence of the upper limbs, which are raised from the ground and relieved of the function of locomotion—a function that still continues in all other mammals, excepting the anthropoid apes, whose upper limbs are extremely long and barely escape the earth, and serve the animal merely as an aid and a support in walking. The birds, although supported on their hind limbs alone, nevertheless have their fore limbs assigned to the sole office of wings for the transportation of their bodies.
Consequently, the free and disposable upper limb, peculiar to mankind, would seem to mark a new function in the biologic scale—human labour.
Anatomy of the Skeleton of the Limbs.—In contrast to the bust, the limbs have an internal skeleton, adapted solely to the function of support (not of protection). The bones are covered with masses of striped muscles, which have as their special function voluntary movement, that is to say, obedience to the brain.
The upper and lower limbs correspond numerically, and the arrangement of the bones is analogous; and this holds true for all the higher vertebrates. The nearest bones, those that are attached to the trunk, are single in all four limbs. Then, just as though branching out, they next double in number, and then multiply successively as we approach the extremities of the limbs. Thus the forearm and the lower leg have two bones, and the hands and feet have many.
In the upper arm we have the humerus, in the thigh the femur, in the forearm the ulna and radius (the ulna is situated on the same side as the little finger and the radius on that of the thumb), in the lower leg the tibia and fibula. Then come the many short bones (eight in the carpus and seven in the tarsus) which in the hand form the wrist or carpus, and in the foot the ankle or instep, the tarsus. These are followed by other long bones (five in the hand and five in the foot), which constitute the metacarpus and metatarsus, and these in turn by the long bones of the phalanges (fingers and toes), which grow successively smaller toward the extremities and are successively named proximal, middle and distal phalanges (phalangettes). These last are missing in the thumb and the big toe. In conjunction with the last phalanges, the fingers and toes are protected by nails.
The Growth of the Limbs.—Recent studies, conducted principally by Godin in France, author of the classic work upon growth, have demonstrated that the long bones of the limbs obey certain special laws of biologic growth.
While a long bone is growing in length it does not grow in width or thickness, and while it is increasing in thickness it does not gain in length; hence the lengthening of the bones takes place in alternate periods; during the period of repose relative to growth in length, the bone gains in thickness.
I have already explained, in connection with the stature, that we owe the growth of the long bones to a variety of formative elements, the cartilages of the epiphyses, which control the growth in length of the long bones, and the enveloping membrane of the body of the bone, the periosteum, which presides over the growth in thickness.