In those of the vertebrated animals which have no lymphatic glands, the thorough inter mixture of the fluids contained in the lymphatic vessels is provided for by numerous plexuses,t and, in the case of reptiles, by distinct lympha tic hearts communicating with veins ;§ and we are sure, that much of the matters absorbed in these animals, whether by veins or lymphatics, passes through the capillaries of the kidneys or liver, as well as the lungs, before reaching the arteries.
When we see so much contrivance, evidently adapted for giving every facility to the gradual operation of the vital affinities subsisting among the constituents of the blood, before it reaches the scene of any of the acts of nutrition, secre tion, or excretion, we cannot be surprised to find, that these acts themselves should appear to be so simple as the observations already quoted would seem to indicate.
It must be admitted, that if we consider these contrivances in the higher animals as important agents in the elaboration of the blood, and con sequent folmation of the textures and prepared fluids of the body, there is a difficulty in under standing how these objects can be accomplished in the lowest classes, particularly the insects and zoophyta, where the nourishment of various textures, and formation of secretions and tions, has been thought to be merely in the way of imbibition from a central cavity.• But it is to be observed, that in several of these tribes, in insects, and even in the infusory animals, recent observations have disclosed a much more plex apparatus for the movement of the fluids, than was previously suspected. And, in regard
to the lowest zoophyta, it may be said in general, that if there is little apparent provision for the elaboration of the fluids, there is also little occasion for it,—first, because there is little variety of textures to be nourished, and secondly, because the simplicity of their structure is such, that all the particles of their nourishing fluid, — admitted into a central cavity, flowing thence towards their surface, and acted on by the air at all parts of that are similarly situate in regard to all the agents by which they can be affected, and must be equally fitted for the changes which the vital affinities there acting on them can produce, so that the same necessity for gradual intermixture, and repeated agitation, of heterogeneous rials, does not probably exist in them, as in the animals of more complex structure. The analogy of their economy, therefore, is not a serious objection to the inference we have drawn from so many other facts, as to the numerous changes which are wrought in the blood of the higher animals, while circulating in the vessels, and as to the function of tion being a necessary accompaniment of the assimilation of aliment, and nutrition of tures, even independently of their renovation by processes of ultimate deposition and absorption. ( IV. P. Alison.)