Popular Science Monthly/Volume 52/January 1898/Feet and Hands I
|FEET AND HANDS.|
By M. BERNARD (Mrs. HENRY BERNARD).
IN the great family of the backboned animals, to which we ourselves belong, many different kinds of feet and hands are to be found, their shape showing a very wonderful relation to the manner of life of their possessors.
We are not going here to describe the fins of fishes, although many people believe that our feet and hands were developed from fins; we shall only deal with the true feet and hands found in animals higher than the fishes.
When we try to picture the foot or hand of any animal, we naturally think of it as we see it, often covered with fur which to a great extent hides its real shape. In our own feet and hands the true shape is more evident because the skin has lost its hairy covering. To obtain a clear idea of the feet and hands of different animals it is therefore better to limit ourselves to the bones which give them their firmness.
However different feet and hands may be, there is a certain remarkable similarity of plan in them all. In all, as in our own, there is a cluster of bones forming an "ankle" or a "wrist," and then running out from this a certain number of what we may call "rays." In each foot and hand there were once five rays, as there now are in our own, but in many animals this original number has been reduced, as we shall see. Each ray consists of several joints; the first joints are the longest, and are bound together by skin and flesh to form the sole of the foot or the palm of the hand, while the other joints form the free toes and fingers, the skin at their tips carrying nails or claws.
We can all trace this general plan both in our own feet and in our own hands, although in outward form our feet do not resemble our hands, and the work they have to do for us is so very unlike that it may seem surprising they should be built upon the same plan.
The explanation of this similarity of plan is very simple. Once, very long ago, there was no difference in the work done by feet and hands; they were all used for walking, and were thus all, properly speaking, feet. So if we want to understand our feet and hands we must go back to animals which had four feet and no hands, the four feet each having five toes.
Fig. 1 is a drawing of the foot of a crocodile, which is probably not unlike the earliest kind of foot. At the end of the leg bones a and b we see several small bones, and, starting from these, five jointed rays which in the natural position lie flat on the ground. The fore feet and hind feet of the crocodile are almost exactly alike, having the same work to do—i.e., helping the animal to shuffle along the ground. This kind of foot is very well suited for reptiles, such as crocodiles and lizards, which lead a more or less lazy life, merely moving from place to place to find a patch of sunshine to lie in, or a spot which the animals or insects they feed on frequent, and where they can be snapped at easily. Some of these animals, it is true, are capable of darting at times with lightning speed, but this they seem to do by the help of their tails.
The clumsy, shuffling way of walking on the flat of the foot has been given up by most animals, but we still find it in some, such as the bears, which are called plantigrades or sole-walkers because of their flat-soled feet. The feet of the bear are very superior to those of the crocodile, for they are armed with claws which help him to hold prey and to climb trees; but his awkward, shuffling gait shows pretty plainly that the method of moving on the flat sole is not the best possible for running.
A better method of running is found in most of the higher animals, which no longer touch the ground with the sole of the foot, but only with the toes or digits, and are therefore called digitigrades or toe-walkers. Fig. 2 is the foot of a doglike animal (the wolf), and Fig. 3 is that of a catlike animal (the lion), and in both of these we see that the part of the foot which in ourselves we call the heel does not lie on the ground as in the crocodile or the bear, but is raised high above it, so that the animal walks, as we have said, on its toes or digits. The reason why this is a better method of running is that it gives extra length to the leg. A longer leg means a longer stride and greater speed. The wolves, hyenas, and foxes, which are doglike animals, and the lion, leopard, lynx, and all other catlike animals are beasts of prey—that is, they eat other animals which they have to hunt—and all in their turn are liable to be hunted, so they need to be able to run very swiftly. Many of the catlike animals, too, depend, both in attack and defense, quite as much on springing as on running, and so need to be very agile. For both running and springing we ourselves even use our toes and not the soles of our feet, and these animals, with whom running and springing have become a constant habit, have come to use only their toes always; the heel no longer touches the ground except when the animal is crouching.
Now, in the foot of the crocodile (Fig. 1) it is seen that all the toes are not of the same length. If animals with such feet began to walk on their toes, some would not touch the ground. This, indeed, is what we find in many animals, especially in those cases where running is all that is required. In the feet of the dog, for instance,
|Fig. 1.||Fig. 2.||Fig. 3.||Fig. 4.|
only four toes touch the ground; the fifth toe, having long been less used than the others, has become very small. In the hind feet, which are used exclusively for running, the remains of the fifth toe can be felt as a small projection under the skin, some way up the back of the leg. In the fore foot of the dog, however, it is a distinct toe, with a nail, which is still used for digging.
In cats and dogs, as in the bears, claws are well developed. In the dogs they are short and hard but not very sharp, and are used chiefly for digging. The catlike animals, on the contrary, have very sharp, hooklike claws, which are of great use in seizing and holding prey.
It is obvious that if an animal, changing still further its method of walking, took to running on the tips of its nails or claws instead of on its toes, it would have a still greater length of leg, and therefore would be still better fitted for running swiftly. This remarkable manner of running is actually found in most of the swiftest four-footed animals, such as the horses and deer, and also in the cows, the sheep, and the pigs, which are therefore called ungulates, ungula meaning a nail. All these animals feed chiefly on grass, of which they need great quantities to nourish their usually large bodies. To obtain constant supplies of grass, in a wild condition, they have continually to roam from place to place. Their feet are admirably suited for these roaming habits, and also for very swift running, which is their best chance of escape from their enemies, the flesh-eating wolves, tigers, etc.
The foot of the bison (Fig. 4), in which the tips of only two toes touch, the ground, shows very clearly the position assumed by the bones of the foot in these animals. Not only are the heel and the first long joints raised above the ground, as in the dogs and cats, but the two remaining long upper joints join together to form one strong bone (Fig. 4 *), and none of the lower joints of the toes touch, the ground. The nails of two toes which form what we call the "hoof" rest upon it, while at a we see all that is left of a third and fourth toe, possessed by the distant ancestors of the bison.
The chief distinction between the feet of the different nail-walkers is the number of toes whose nails form the hoof. Fig. 5 A shows us that the pig has four toes, but only two touch the ground and make the hoof; the other two are useless, and are gradually becoming smaller. The rhinoceros (Fig. 5 B) has only three toes, and these are not all equal in length, but all are in use and end in massive hoofs. The cow (Fig. 5 C) has only two toes, the upper joints of which have grown together into one as in Fig. 4 (for the bison is one of the cows) so as to form a long, strong part of the leg, the two very
thick nails making a double hoof. This is what is called in the Bible a "cloven" hoof; it is not, however, as was supposed, a single nail split down, but two distinct nails belonging to two toes. Then, lastly, the horse (Fig. 5 D) has lost all its toes except one, which has become exceedingly thick and strong. The horse on its four feet, each, ending in only one toe with its great nail, attains immense speed in running, great length being given to its stride by the lengthening out of the one bone (*), which remains as the representative of the "sole" of the foot. Ages ago, as is proved by fossil remains, the forefathers of the horse of to-day had three, four, or even five toes. At a, a, on each side of the great single bone two thin bones can be seen. These two bones are the splint bones, the only remains now to be found of the vanished toes.
All the animals we have so far mentioned have four feet more or less alike, all being used for the same purpose, that of running. But some animals came to use their fore feet for one purpose and their hind feet for another, and in consequence of this the fore feet came to be unlike the hind feet.
A striking example of such a difference in the use of the fore and hind feet, leading also to a difference in their structure, is found in the kangaroo, an animal which is seen wild only in Australia, where it hops or leaps over the open country, more or less upright, with extraordinary swiftness by means of its hind feet alone. When it rests, it sits on the long soles of its hind feet, steadied by its thick tail. Fig. 6 A represents the hind foot of a kangaroo in its ordinary position when leaping, and Fig. 6 B shows of what strangely changed bones it is composed. One ray has become very long and thick, and another, though not so long, is also fairly thick, but the other two are quite thin, as if they were dying away from not being used.
The fore feet of the kangaroo (Fig. 6 C) are never used as walking feet except when the animal is hobbling about slowly. Their chief work is grasping and tearing the leaves, grass, or fruit, or digging up the roots which form the food of the kangaroo. They are never needed to support the weight of the body, and so the toes are not large and thick, and four of the five toes are kept. In looking at the fore feet of the kangaroo one is tempted to call them "hands," for this very interesting difference in the use of fore and hind limbs in other animals, such as the monkeys, gave rise gradually to a true grasping hand.
In a second paper we shall deal with the extraordinary transformations of the fore feet into paddles and wings, found in whales, bats, and birds, and shall also see how true hands came to be developed in monkeys and in man.