LEECHES, MEDICAL USE OF. Of the species described in the article ANNELIDA, in NAT. HIST. DIY., it is intended to treat here only of those of the genus Sanguisuge (Savigny), or latrobdella (Blainville), as they only can bo employed for medical purposes. The same reason induces us to confine our attention to the species S. qificinalis (Savigny) and S. medicinalis (Savigny).
Though the S. obscure and S. interrupta might be employed to with draw blood, yet the S. officinalis and S. medicindis are chiefly so used. The former is also termed the Hungarian or green leech (Ilirudo pro rincialis, of Carena ; S. meridionalis, of Risco), while the latter is termed the German, or brown, or gray leech, also the true English or specked leech. The one species abounds in the south of Europe, while the other is a native of the north. The S. medicinulia is now rare in England, owing to the draining of so many of the ponds and bogs in which it formerly abounded. The same is nearly the case in France, which used to be supplied chiefly from the district of La Brenne, but now from the frontiers of Russia and Turkey. England derives the immense number required mostly from Sweden, Poland, and Hungary.
The genus Sanguiauga is characterised by having the body elongated, the back convex, the belly flat, and the oral and caudal extremities narrowed, before they spread 'out into discs or suckers. The body consists of from ninety to one hundred or more soft rings, which do not increase in number, but only in size, with the age of the animal, which requires about eight years to come to maturity ; and if it escape being devoured by others which prey upon it, it may attain twenty years. The anterior or oral extremity is rather narrower than the caudal : it is provided with ten blackish points or eyes, and a triradiate (not triangular) mouth, furnished with three cartilaginous jaws, each armed with numerous cutting-teeth. The anus is very small, situate on the dorsal surface of the last ring.
The S. of inetis has a green body or light blackish-green, the back marked with six longitudinal bands of an iron colour, spotted with black spots at their middle portion and edge. The belly is of a yellowish. green without spots, but broadly bordered with black. The segments of the body are very smooth. It is large, often seven inches long. It lives in pools and rivers. There are three varieties.
S. medicinalis has the body of a deep green, its back marked with six longitudinal bands of an iron colour, pretty clear, spotted:with black points, generally triangular. The belly is greenish, spotted, and broadly bordered with black,' and the segments of the body rough from granular eminences. It inhabits ponds and small lakes.
Qf the anatomy of the leech it is not necessary to say much. Th( skin consists of two layers, the external or epidermis, and the internal or corium. The first is transparent, resembling a germ membrane ; this is throWn off from the body every four or five days. The corium consists of condensed cellular tissue. It displays the divisions into rings, and in it resides the colouring matter of th( leech.
The alimentary system consists of the mouth, the stomach, salivary glands, liver, and anus. The mouth has a triradiate figure, formed o three equidistant lines, meeting in a centre, about the middle of tin oral disc. Inside are three sublenticular jaws or piercers, white, ant of a cartilaginous appearance. On the free, curved, sharp margin o each jaw there are about sixty small fine-pointed teeth. The alimentary canal consists of an oesophagus, a long stomach, with meal sacs, an( ,n intestine. The oesophagus is a muscular tube, and commences )etween the inner angle of the three jaws by a roundish opening : t dilates as it approaches the stomach, but at its termination it con racts into a circular aperture : the whole length does not exceed a luarter of an inch. The stomach occupies two-thirds of the length of he animal, and is formed of eleven compartments or cella. Each of ,hese divisions, that is, from the second to the eleventh, gives off on acli side a sac, of which those of the last cell are much the largest. the intestine is about an inch in length ; at the upper orifice is a ,alve, and at its lower a sphincter. These organs can contain nearly aalf an ounce of blood ; so that there is nothing remarkable in the statement that leeches have been known to exist three years in water, without any other nourishment than they could obtain from it ; for the blood is received into cells quite distinct, in the first eight of which it remains for months, without undergoing any change either in colour or fluidity : over these cells the animal has a perfect control, merely allowing so much nutriment to pass into the alimentary canal as is necessary to preserve its existence. This accounts for the reluctance of the animal, after being used to abstract blood, to repeat the opera tion ; it not only being gorged at the time, but provided with nutriment sufficient to serve it during almost a sixth portion of its life. In its native abode the true medicinal leech does not seem to take any solid aliment, but subsists on the fluids of fish, frogs, &c.