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and meeting in the middle of its cavity,- strut up the channel. Can any one doubt of contrivance here; or is it possible to shut our eyes against the proof of it?
This valve also, is not more curious in its structure, than it is important in its office. Upon the play of the valve, even upon the proportioned length of the strings or fibres which check the ascent of the membranes, depends, as it mould seem, nothing less than the life itself of the animal. We may here likewise repeat,what we before observed concerning some of the ligaments of the body, that they could not be formed by any action of the parts themselves. There are cases, in which, although good uses appear to arise from the shape or configuration of a part, yet that shape and configuration itself may seem to be produced by the action of the part, or by the action or pressure of adjoining parts. Thus the bend, and the internal smooth concavity of the ribs, may be attributed to the "equal pressure of the soft bowels; the particular shape of some bones and joints, to the traction of the annexed muscles, or to the position of contiguous muscles. But valves could not be so formed. Action and pressure are all against them. The blood, in its proper '• course, course, has no tendency to produce such things; and, in its improper or reflected current, has a tendency to prevent their production. Whilst we see, therefore, the use and necessity of this machinery, we can look to no other account of its origin or formation than the intending mind of a Creator. Nor can we without admiration reflect, that such thin membranes, such weak and tender instruments, as these valves are, should be able to hold out for seventy or eighty years.
Here also we cannot consider, but with gratitude, how happy, it is that our vital motions are involuntary. We should have enough to do, if we had to keep our hearts beating, and our stomachs at work. Did these things depend, we will not fay upon our effort, but upon our bidding, our care, or our attention, they would leave us leisure for nothing else. We must have been continually upon the watch, and continually in fear : nor would this constitution have allowed of steep.
It might perhaps be expected, that an organ so precious, of such central and primary importance, as the heart is, should be defended by a case. The fact is, that a membranous purse or bag, made of strong tough materials,
5" is provided for it; holding the heart within its cavity; fitting loosely and easily about it; guarding its substance, without confining its motion; and containing likewise a spoonful or two of water, just sufficient to keep the surface of the heart in a state of suppleness and moisture. How should such a loose covering be generated by the action of the heart? Does not the inclosing of it in a fac, answering no other purpose but that inclosure, shew the care that has been taken of its preservation?
One Use of the circulation of the blood (probably amongst other uses) is to distribute nourishment to the different parts of the body. How minute and multiplied the ramifications of the blood-vessels, for that purpose, are; and how thickly spread, over at least the superficies of the body* is proved by the single observation, that we cannot prick the point of a pin into the flesh, without drawing blood, i. e• without finding a blood-vessel. Nor, internally, is their diffusion less univerfal. Bloodvessels run along the surface of membranes, pervade the substance of muscles, penetrate the bones. Even into every tooth, we trace, through a small hole in the root, an artery to feed the bone, as well as a vein to, bring back the spare blood from it; both which, with the addition of an accompanying nerve, form a thread only a little thicker than a horsehair.
Wherefore, when the nourishment taken in at the mouth, has once reached, and mixed itself with, the blood, every part of the body is in the way of being supplied with it. And this introduces another grand topic, namely, the manner in which the aliment gets into the blood, which is a subject distinct from the preceding, and brings us to the consideration of another entire system of vessels.
II. For that necessary part of the animal economy, an apparatus is provided, in a great measure, capable of being, what anatomists call, demonstrated, that is, shown in the dead body; and a line or course of conveyance, which we can pursue by our examinations.
First, the food descends by wide passages into the intestines, undergoing two great preparations on its way, one, in the mouth by mastication and moisture, (can it be doubted with what design the teeth were placed in the road to the stomach, or that there was choice in fixing them in this situation ?) the other, by digestion in the stomach itself. Of this last
N surprising surprising dissolution I fay nothing; because it is chymistry, and I am endeavouring to display mechanism. The figure and position of the stomach (I speak all along with a reference to the human organ) are calculated for detaining the food long enough for the action of its digestive juice. It has the shape of the pouch of a bagpipe; lies across the body; and the pylorus, or passage by which the food leaves it, is somewhat higher in the body, than the cardia or orifice by which it enters; so that it is by the contraction of the muscular coat of the stomach, that the contents, after having undergone the application of the gastric menstruum, are gradually pressed out. In dogs and cats, this action of the coats of the stomach has been displayed to the eye. It is a slow and gentle undulation, propagated from one orifice of the stomach to the other. For the-fame reason that I omitted, for the present, offering any observation upon the digestive fluid, I shall fay nothing concerning the bile or the pancreatic juice, further than to observe upon the mechanism, viz. that from the glands in which these secretions are elaborated, pipes are laid into the first of the intestines, through which pipes the product of each gland flows