Fig. [2].
Seal’s Foot.
Fig. [3].
Lizard’s Foot
It is a curious and interesting fact, affording a remarkable illustration of the close adherence to a uniform plan which has been observed in the construction of the various animals, that, in no instance, does this toe contain more than two bones. Even in those creatures, as the Seal (fig. 2), in which it attains to greater length than any of the other sprawling digits, it contains the same number of bones as in man, its extraordinary length being attained by an elongation of the two bones, not by the addition of a third. And in those animals, as certain Lizards (fig. 3), where the number of bones in the other toes is increased to 4 or even 5, the number in the first, or inner, toe is still no more than two. The same rule applies to the fore limb; the number of bones in the inner digit, which, in man and monkeys, is called the “thumb,” is in no case more than two. In some animals, as will be mentioned again, there is only one bone in this digit, and in some the digit is wanting altogether; but in none does it contain more than two bones.
This reminds me of a still more remarkable instance of adherence to a particular number of bones. In the mammalian group of animals the neck, with only one or two exceptions, contains seven bones, neither more nor less. Whether it be the long neck of the Giraffe, or the short neck of the Mouse, the Bat, or the Porpoise, each consists, like the neck in Man, of seven bones. For what reason a particular number should be thus rigidly observed, it is not easy to say.
Of the seven tarsal bones the uppermost (fig. [1]) is called the astragalus, from a supposed resemblance to a die. It is the middle bone of the instep. Above, it is jointed with the leg-bones; behind, it is connected with, and rests upon, the heel-bone, which is the largest bone in the foot. The bone which lies immediately in front of the astragalus, and supports it in this direction, is called the scaphoid, or boat-like, bone. In front of it are three wedge-bones, each of which is connected with one of the metatarsal bones of the inner three toes. On the outer side of the wedge-bones, connected with the metatarsals of the two small toes, and locked in between them and the heel-bone, is the cuboid bone.
I must confine my remarks chiefly to the human foot. Still the anatomy of man derives so much interest from being studied in connexion with that of the lower animals, and is so much more instructive when this is done, that I cannot forbear diverging, here and there, to make a few comparisons. Let me, for a moment, draw your attention to a similarity, in general construction, which exists between the lower limbs of man, and the hinder limbs of other animals. And the comparison may be extended to the fore limbs; for however diverse may be the appearance and the mode of action of the limbs in different animals, whether they be terminated by hands or by feet, whether they move upon the ground or ply in air or water, whether they be attached to the head, as are the front fins in many fishes, or, as is more common, be situated at the fore and hinder parts of the trunk, the same plan is traceable in all.
Great, indeed, is the variety of detail in nature. It is everywhere observable. No two things, however near their resemblance, are precisely alike. Yet, as I have before said, there is a remarkable adherence to unity of plan. One star differs from another star in glory, yet all appear fashioned in the same manner, and subject to the same laws. There are almost infinite varieties in the vertebrate kingdom. Each animal exhibits its own peculiarities; yet they are all formed in the same manner, and are developed upon one fundamental pattern, diverging from it in different ways according to the requirements of each. Again, though the several parts of the same animal differ from one another; yet in the skeleton the same bones which exist in one part may, as a general rule, be traced in other parts and in other animals. The bones which make up the pelvis in man are repeated in his shoulder, and, even, in his skull; and they may be recognised in the pelvis, in the shoulder, and in the skull, of all other vertebrate animals, with few exceptions. They undergo, it is true, great varieties in shape and size; but they can be shown to be the same, or, in the language of anatomists, to be “homologous.” It is highly interesting to the anatomist to trace the same bone through the different parts of the same animal, and through the various animals of the vertebrate series, and to observe the modifications which it undergoes in order to adapt it to the multiform mechanism of the several classes, to observe it sometimes dwindling, or even vanishing, and then, it may be, reappearing under some new conditions.
Fig. [4].
Human Leg