But to support, to give firmness and shape to the fabric, to keep the softer parts in their proper places, to give fixed points and the proper direction to its mo tions, as well as to protect some of the more important and tender organs from external injuries, there must be some firm prop-work interwoven through the whole. And, in fact, for such purposes the bones are given.
. The prop-work must not be made into one rigid fabric, for •that would prevent motion. l'herefore there are a number of bones. These pieces must all be firmly bound together, to prevent their disloca tion, and this end is perfectly well an swered by the ligaments. The extreini ties of these bony pieces, where they move and rub upon one another, must have smooth and slippery surfaces, for easy motion. This is most happily pro vided for by the cartilages and mucus of the joints.
The interstices of all these parts must be filled up with some soft and ductile matter, which shall keep them in their places, unite them, and at the same time allow them to mCrie a little upon one an other. This end is accol dingly answered by the cellular membrane, or adipous substance.
There must be an outward covering over the whole apparatus, both to give it atom compactness, and to defend it from a thousand injuries, which, in ffict, are the very purposes of the skin, and other tegtunents.
As she is made for society and inter course with beings of her own kind, she must be endued with powers of expres sing and communicating her thoughts hy some sensible marks or signs, which shall be both easy to herself, and admit of great variety. Hence she is provided with the organs and faculty of speech, by which she can throw out signs with amazing fa cility, and vary them without end.
Thus we have built up an animal body, which would seem to be pretty complete , but we have not yet made any provision fin its duration : and, as it is the nature of matter to be altered and worked upon by matter, so in a very little time such a ing- creature must be destroye d, if there is no provision for repairing the injuries which she must commit upon herself, and the injuries which she must be exposed to from without. Therefore a treasure of blood is actually provided irr the heart and vascular system, full of nutritious and healing particles, fluid enough to pene trate into the minutest parts of the animal.
Impelled by the heart, and conveyed by the arteties, it washes every part, builds up what was broken down, and sweeps away the old and useless materials.
Ilence we sec the necessity or advan tage of the heart and arterial system: the overplits of this blood, beyond what was required 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 venal system is actually provided. These requisites in the animal explain, a priori, the circula tion of the blood.
The old materials, which are become useless, and are swept off by the current of blood, must be separated and thrown out of the system. Therefore glands, the organs of secretion, are given, fin strain ing Whatever is redundant, vapid, or noxi ous, from the mass of blood : and, when strained, it is thrown -out by excre tories.
Now, as the fabric must be constantly wearing, the repa.ration must be carried on without intermission, and the strainers most always be employed: therefore there is actually- a perpetual circulation of the blood, and the secretions are a ways going on.
But even all this prcrvision would not be sufficient ; for that store of blood would soon be consumed, and the fabric would breakdown, if there were not a provision made for fresh supplies. These we ob serve, in fact, are profusely scattered TOUnd her in the animal and vegetable • kingdoms; and she is provided with hands, the finest instruments that could have been contrived for gathering them, and for preparing them in a variety of diffe rent ways for the mouth. These snIsphes, 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 in to the blood, and the dregs must be thrown off. With this view the intestinal canal is constructed. It separates the nutritious part, which we call chyle, to be conveyed into the blood by the system of absorbent vessels; and the feces pass downwards, to be conducted out of the body.