What more there was of the blood than enough to repair the present damages of the machine, must not be lost, but should be returned again to the heart; and for this purpose the venous system is provided. These requisites in the animal explain the circulation of the blood, a priori.[1]
All this provision, however, would not be sufficient; for the store of blood would soon be consumed, and the fabric would break down, if there was not a provision made by fresh supplies. These, we observe, in fact, are profusely scattered round her in the animal and vegetable kingdoms; and she is furnished with hands, the fittest instruments that could be contrived for gathering them, and for preparing them in their varieties for the mouth.
But these supplies, which we call food, must be considerably changed; they must be converted into blood. Therefore she is provided with teeth for cutting and bruising the food, and with a stomach for melting it down; in short, with all the organs subservient to digestion: the finer parts of the aliments only can be useful in the constitution; these must be taken up and conveyed into the blood, and the dregs must be thrown off. With this view, the intestinal canal is provided. It separates the nutritious parts, which we call chyle, to be conveyed into the blood by the system of the absorbent vessels; and the coarser parts pass downwards to be ejected.
We have now got our animal not only furnished with what is wanting for immediate existence, but also with powers of protracting that existence to an indefinite length of time. But its duration, we may presume, must necessarily be limited; for as it is nourished, grows, and is raised up to its full strength and utmost perfection; so it must in time, in common with all material beings, begin to decay, and then hurry on into final ruin.
Thus we see, by the imperfect survey which human reason is able to take of this subject, that the animal man must necessarily be complex in his corporeal system, and in its operations.
He must have one great and general system, the vascular, branching through the whole circulation: another, the nervous, with its appendages—the organs of sense, for every kind of feeling: and a third, for the union and connection of all these parts.
Besides these primary and general systems, he requires others, which may be more local or confined: one, for strength, support, and protection,—the bony compages: another, for the requisite motions of the parts among themselves, as well as for moving from place to place,—the muscular system: another to prepare nourishment for the daily recruit of the body,—the digestive organs.
Dr. Paley observes, that, of all the different systems in the human body, the use and necessity are not more apparent, than the wisdom and contrivance which have been exerted, in putting them all into the most compact and convenient form: in disposing them so, that they shall mutually receive from, and give helps to one another: and that all, or many of the parts, shall not only answer their principal end or purpose, but operate successfully and usefully in a variety of secondary ways. If we consider the whole animal machine in this light, and compare it with any machine in which human art has exerted its utmost, we shall be convinced, beyond the possibility of doubt, that there are intelligence and power far surpassing what humanity can boast of.
One superiority in the natural machine is peculiarly striking.—In machines of human contrivance or art, there is no internal power, no principle in the machine itself, by which it can alter and accommodate itself to injury which it may suffer, or make up any injury which admits of repair. But in the natural machine, the animal body, this is most wonderfully provided for, by internal powers in the machine itself; many of which are not more certain and obvious in their effects, than they are above all human comprehension as to the manner and means of their operation. Thus, a wound heals up of itself; a broken bone is made firm again by a callus; a dead part is separated and thrown off; noxious juices are driven out by some of the emunctories; a redundancy is removed by some spontaneous bleeding; a bleeding naturally stops of itself; and the loss is in a measure compensated, by a contracting power in the vascular system, which accommodates the capacity of the vessels to the quantity contained. The stomach gives intimation when the supplies have been expended; represents, with great exactness, the quantity and quality, of what is wanted in the present state of the machine; and in proportion as she meets with neglect, rises in her demand, urges her petition in a louder tone, and with more forcible arguments. For its protection, an animal body resists heat and cold in a very wonderful manner, and preserves an equal temperature in a burning and in a freezing atmosphere.
A farther excellence or superiority in the natural machine, if possible, still more astonishing, more beyond all human comprehension, than what we have been speaking of, is the distinction of sexes, and the effects of their united powers. Besides those internal powers of self-preservation in each individual, when two of them, of different sexes, unite, they are endued with powers of producing other animals or machines like themselves, which again are possessed of the same powers of producing others, and so of multiplying the species without end. These are powers which mock all human invention or imitation. They are characteristics of the Divine Architect.—Thus far Paley.