VOL. XXIII.] PHILOSOPHICAL TRANSACTIONS. 707 



Her here laid down. Bat though our bodies be entirely composed of tubuli, 

 or vessels filled with fluids, yet this pressure, how great soever, being equal, 

 could have no effect upon them, if the superficial dimensions were not easily 

 variable ; because, being compressed on all parts with the same degree of force, 

 the contained fluids could not any where begin to recede, and make way for 

 the rest to follow, but would remain as fixed and immoveable as if they were 

 actually solid. But by the dilatation of the thorax, room is made for the fluids 

 to move ; and by the coarctation of it fresh motion is impressed ; which is the 

 main spring by which the circulation is set and kept going. 



This reciprocal dilatation and contraction of the superficial dimensions of the 

 body, seem so necessary to animal life, that there is not any animal so imper- 

 fect as to want it, at least none to the inward structure of which our anatomi- 

 cal discoveries have yet reached. For though most kinds of fish and insects 

 want both moveable ribs, and lungs, and consequently have no dilatable thorax, 

 yet that want is made up to them by an analogous mechanism, answering 

 sufficiently the necessities of their life. Those fishes, which have no lungs, 

 have gills, which do the office of lungs, receiving and expelling alternately 

 the water, by which the blood vessels suffer the same alteration of dimensions 

 that they do in the lungs of more perfect animals. 



The lungs or air vessels of insects are yet exceedingly more different in 

 structure, distribution, and situation, from those of perfect animals, than 

 those of fishes are; and yet in their use and action agree perfectly with 

 both, that is, receiving and expelling the air, and varying the dimensions 

 and capacities of the blood vessels. These having no thorax, or se- 

 parate cavity for the heart and air vessels, have the latter distributed through 

 the whole trunk of their bodies, by which they communicate with the external 

 air through several spiracula or vent holes, to which are fastened so many little 

 tracheas, or wind-pipes, which thence send their branches to all the muscles, 

 and viscera, and seem to accompany the blood vessels all over the body, as 

 they do in the lungs only of perfect animals. By this disposition in every in- 

 spiration, the whole body of these little animals is inflated, and in every expi- 

 ration compressed, and consequently the blood vessels must suffer a vicissitude 

 of extension and contraction, and a greater motion must thereby be impressed 

 on the fluids contained in them, than the heart, which does not in these ani- 

 mals appear to be muscular, seems capable of giving. 



The only animal that is exempted from this necessary condition of breath- 

 ing, or receiving and expelling alternately some fluid into and out of the body, 

 is a foetus. But this, while included in the womb, has little more than a 

 vegetative life, and ought scarcely to be reckoned among the number of animals. 



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