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 separate 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 fasten'd so many little Tracheæ , 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 Inspiration , the whole Body of these little Animals is inflated, and in every Expiration compress'd; and consequently the Blood-Vessels must suffer a Vicissitude of Extension and Contraction, and a greater motion must thereby be impress'd upon the Fluids contain'd in them, than the Heart, which does not in those Creatures appear to be Muscular, seems capable of giving.
The only Animal that is exempted from this necessary condition of Breathing, or receiving and expelling alternately some Fluid into and out of the Body, is a Fœtus. But this, while included in the Womb, has little more than a vegetative Life, and ought scarce to be reckon'd among the number of Animals. For, were it not for that small share of Muscular Motion, which it exercises in the Womb, it might without absurdity be accounted for as a Graft upon, or Branch of the Mother.
Concerning the immediate Matter, and Means of Life, and Nutrition, Authors are not agreed, nor is it the business of this place to reconcile, or decide their Differences, but to account for the Motion of the Blood through the Vessels only. In order to this, it will be necessary to observe, that the Pulsation of the Heart in a Fœtus is so very weak and obscure, and the Motion of the Blood so extream slow and languid, as to be scarce,