Page:The Zoologist, 1st series, vol 1 (1843).djvu/121

Rh ones. Durondeau has mentioned the presence of a heart, or central organ of circulation, in the medicinal leech. He describes it as a fleshy pouch, of a conical figure, attached to the back by large vessels but having its apex free. This statement has not been confirmed by any other author; and in all my examinations of the horse-leech, I have never been able to. detect such an organ.

The lateral vessels (fig. a a) run from one extremity of the body to the other, in a wavy course, forming a series of festoons. Some authors describe them as quite straight when the animal is in motion or stretched out, but wavy when the animal is at rest in its constricted state: I have always found them, both in the horse-leech and the medicinal species, to form a series of festoons in every position of the animal, whether the body was dilated by the distention of the alimentary canal with injection, or when in a state of contraction produced either by alcohol or other corrugating fluids. These vessels are largest in the middle of the animal, and diminish gradually in size as they approach the extremities, at which points they become continuous with one another, and here they give off numerous branches to supply the locomotive sucking disk, the mouth, and the organs of sight. As these lateral trunks proceed in their wavy course, they send off branches (dorso-lateral and abdomino-lateral) at nearly equal intervals; by means of these numerous anastomoses, when injection is forced into one lateral vessel, the vessel of the opposite side, and the ventral and dorsal, become filled as well. On opening a leech a well-marked systole and diastole can be seen in the vessels above described; and as no heart can be detected, we must conclude that these vessels, which are so much larger in diameter and thicker in their parietes than either the dorsal or ventral trunks, must perform the function of propelling the blood through the system, in which respect they are analogous to the dorsal vessel of insects. Dr. Rawlings Johnson counted the pulsations, and found that in the first minute there were ten, in the second nine, and in the third eight; they then became irregular and indistinct, and the leech died.

On a careful examination of the lateral vessels by the microscope, the truth of this assertion is fully borne out by the structure then displayed. If a small portion of one of these trunks be made the subject of investigation, it will be found that its parietes are to all appearance highly muscular, it is composed in fact of two sets of bands, arranged like the rings of a trachea, which take an oblique direction