But in 1827 a German naturalist named Carus discovered that there were real currents of blood circulating throughout the body, and returning to their point of departure. The observations of Carus were repeated and confirmed by many other naturalists, and we are thus enabled to form a sufficiently exact idea of the manner in which the blood circulates. The following summary of the phenomena of circulation among insects is borrowed from "Leçons sur la Physiologie et l'Anatomie comparée," by M. Milne-Edwards:-- The tube which passes under the skin of the back of the head, and front part of the body, above the alimentary canal, has been known for a long time as the dorsal vessel. It is composed of two very distinct portions: the anterior, which is tubular and not contractile; and the posterior, which is larger, of more complicated structure, and which contracts and dilates at regular intervals. This latter part constitutes, then, more particularly the heart of the insect. Generally it occupies the whole length of the abdomen, and is fixed to the vault of the tegumentary skeleton by membranous expansions, in such a manner as to leave a free space around it, but shut above and below, so as to form a reservoir into which the blood pours before penetrating to the heart. This reservoir is often called the auricle, for it seems to act as an instrument of impulsion, and to drive the blood into the ventricle or heart, properly so called. The heart is fusiform, and is divided by numerous constrictions into chambers. These chambers have exits placed in pairs, and membranous folds which divide the cavity in the manner of a portcullis. The lips of the orifices, instead of terminating in a clean edge, penetrate into the interior of the heart in the form of the mouth-piece of a flute. The double membranous folds thus formed on each side of the dorsal vessel are in the shape of a half moon, and separate from each other when this organ dilates; but the contrary movement taking place, the passage is closed. By the aid of this valvular apparatus, the blood can penetrate into the heart from the pericardic chamber, the empty space surrounding the heart, but cannot flow back from the heart into that reservoir.