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HE main facts of blood circulation have been known only two hundred and fifty years. This would be surprising if we were not aware that most of our certain knowledge in natural history, including many truths of easier discovery than the circulation of the blood, has been gained within the last one hundred years. And, indeed, the blood and its movements are not yet fully understood. Several points which, at first thought, would seem of easy solution, are matters in dispute or confessed mysteries. The purpose of this article is, not to publish new truth or discuss difficult points, but to compactly present the fundamental and interesting facts relating to the circulation in all animals.

The necessity of a circulating nutritive fluid lies in the localizing of the process of digestion. In proportion as digestion and absorption of food become specialized and restricted to certain parts, circulation becomes more important in order to convey that food to the tissues, and carry from the tissues the worn-out material. To maintain the character of the fluid, it must itself undergo constant change, and hence the excretory processes—respiration being the most urgent—which increase the necessity for movement of the fluid. Circulation of the nutritive fluid is the immediate function for upbuilding and repairing the body. It harmonizes the several vegetative functions, and should be regarded as the primary function, to which all the others are subservient.

The amœba, sponge, and tapeworm have no blood; they have no necessity for it, as they are destitute of digestive organs, their food being in immediate contact with all parts of the body: or, we might regard their blood as simply the water or fluid in which the animal is immersed. In animals possessing the simplest digestive cavities, as the jelly-fish and sea-anemone, the blood is merely the dissolved food, corresponding to the chyme of higher animals. In the starfish, sea-urchin, and other invertebrates, having a complete and distinct stomach, the blood is chyle; while in vertebrates the blood is a distinct fluid, chemically very complex, difficult of analysis, and not perfectly understood: structurally, it is essentially the same in all animals—a clear fluid containing organic particles.

The blood contains all the nourishment which supports the various tissues of the whole structure. It may properly be regarded as the fundamental tissue, and is well named in the French chair coulant—running flesh. It changes rapidly by eating, exercise, and any influence which affects the supply of nutriment or the waste of the body. It is derived primarily from the new food, received in the higher