Confined solely to that part which acts continually, the most perfect animal will not appear to differ from those beings to which we can scarcely give the appellation of animal. As to external functions, it would be nearly upon a level with a vegetable; for however different the internal organization of animals and vegetables may be, the inferences will be the same. They each receive nourishment, grow, expand, have external motions, and a vegetating life. But of progressive motion, action, and sentiment, they will be equally destitute; nor be endowed with any interior or apparent character by which animal life may be distinguished. Investing, however, this internal part with senses and members, animal life will presently manifest itself; and the more this cover shall contain of sense and members, the more will the animal life be perfect. It is by this investment that animals differ from each other. The internal part belongs, without exception, to all animals; and is nearly the same in all which have flesh and blood. The external cover, however, is widely different; and it is at its extremities that the greatest differences subsist. In order to elucidate this argument, let us compare the body of a man with that of a horse or an ox. In each the heart and lungs, or the organs of circulation, and of respiration, are nearly the same; but the external cover is highly different. The materials of the animal body, though the parts are similar to those of the human, vary greatly as to number, size, and position; and thereby the dissimilitudes in their respective forms are rendered very wide. Besides, we shall find that the greatest differences are at the extremities; for in dividing the body into three principal parts, the trunk, the head, and the members, we find, that in the head and members, which are the extremities of the body, consist, the most material difference between man and other animals. We discover that the greatest difference in the trunk is at the two extremities; since in men there are clavicles at the upper extremity, which in animals are wanting; and the under extremity of animals is terminated by a tail, consisting of a certain number of exterior vertebræ, which the human body is without. The inferior extremity of the head also, as the jawbones, and the upper extremity, as the bones the forehead, differ prodigiously in man and beast. Finally, by comparing the members of a man with those of other animals, we plainly perceive it is at the extremities they differ most, as no two things bear less resemblance to each other, than the human hand with the foot of a horse or an ox.