A Text book of Physiology

aterials of nutrition to the different parts of the organism, and at the same time carry away some effete products. In the lowest orders of animal life, as the amoebae and infusoria, where no special organs are manifest and no part therefore has needs differing from any other, there is found no circulatory system?no heart or propelling body or any blood-vessels. Its life depends upon diffusion throughout its parenchyma of substances brought from without and of those which must be excreted. It is only as special organs show themselves and the liquids take determined directions toward one or another of them, that blood-vessels are seen to commence; these at the same time become the receptacles of products absorbed for the purposes of nutrition and the distributors of these same materials to the various tissues of the organism. It is, therefore, from complex organisms that the idea of a perfect circulation is gained, with its admirable mechanism for incessant movement whereby the fluid necessary for its growth, functions, and individual life is forced to every part. Viewed as a whole, the vascular system of the higher animals forms a system of branching vessels or canals, closed in all parts, and not showing at any point in their course the least perceptible orifice of communication with the external world. Consequently, the fluids which have to penetrate into the closed channels of circulation, as well as those which have to emerge from them for the needs of s...