Popular Science Monthly/Volume 17/August 1880/The Medicinal Leech
|THE MEDICINAL LEECH.|
MANY swamps and ponds, which are now considered utterly worthless, might be made sources of great profit by devoting them to the production of a worm which is exceedingly valuable, and the cultivation of which requires no expensive outlay. This worm is the medicinal leech; formerly esteemed of no value, and hated and hunted on account of its bloodthirstiness, it has commanded extremely high prices since its useful qualities have been recognized. Its general appearance is familiar, its internal structure is very wonderful. Its body forms a cylindrical sac, composed of a course of about one hundred rings. The terminal ring of the hinder part is broader and stouter than the others, and serves as a foot. At the front extremity, which is more pointed than the hinder part, are two fine, separated lips, which, when brought together, form a closed ring. Several straight lines run along the back for the whole length of the body, while the belly is of a clearer color and is mottled with irregular dark spots. The body of the leech is so elastic that it can stretch itself out to a length of nearly ten inches, and draw itself up again to within the dimensions of an olive. Within and back of the lips are three thick membranous pads covered with a thin, horny mass bearing several rows of microscopic teeth; they may be described as the jaws. Between the jaws passes the very narrow throat, which can be opened and closed at will by means of a transverse muscle. The animal derives its importance to man from the close aggregation of the movable lips, the narrow throat, and the toothed jaws, for it is enabled by this peculiarity to break through the skin and suck the blood from it. The mechanical operation is as follows: When the lips close in a circle upon the air-tight skin, the jaws are also brought down to it and their saw-like teeth are pressed tight upon the cuticle. The throat having now become fast closed, the head of the worm is drawn back a little, and the lips are thereby given the form of an exhausted cupping-glass, which is divided internally, by the jaws still fastened to the skin, into three distinctly separated parts. The skin is powerfully sucked up into these three divisions of the cupping apparatus till it is torn, and rents are formed corresponding to the three spaces between the jaws, the inner ends of which run into each other and form a larger, still three-parted wound. It follows that the sucking of the leech must be without effect on the hairy parts of the body, where a cupping-glass could not be made air-tight, and this is the case. When the space between the skin and lips, which answers to the interior of the cupping-glass, is filled with blood, the throat is opened, the blood is drawn
by sucking movements of the body into the maw, and the mouth of the worm is filled anew with blood. The long, narrow maw is competent, by means of twenty-six peculiarly formed sacs or valves, which are arranged in two rows, to retain an immense quantity of blood without any of it being driven back by the muscular activity of the body; and, if a hole is pricked in the body of the leech at the rear end of the maw, all the blood that has been sucked up may be made to flow out. On account of the narrowness of its throat, the leech can not take solid food. Its usual nourishment consists of animal and vegetable infusoria, which it swallows in masses as the whale does herrings; and while the whale spouts out through its nose the water it has swallowed and only retains the herring, the leech exudes the excess of water by means of a peculiar glandular apparatus in its skin, and keeps the infusoria in its maw. It also readily drinks the blood of cold-blooded and warm-blooded
animals, and fills itself so greedily with the latter that it can not endure the surfeit, and dies soon afterward. Leeches were formerly abundant in the bogs and ponds of Germany, where, by reason of their great fruitfulness, they increased to millions, and were considered so worthless, even noxious, that the owners of the lands permitted the traveling dealers to fish them out at first for nothing, afterward for a small price. Finally the ponds were cleared of them; the dealers had sold the leeches for an immense profit, and millions on millions of them had been exported from Hamburg to America, and wherever else this costly and irreplaceable medical apparatus was needed, while the land of its production had none. The useful leech is not found in all countries, but its abode is limited to central Europe, Asia Minor, and a small part of the northern coast of Africa. In some of these relatively confined regions it has been exterminated. The demand for it has become very great; France and Germany, for example, use about thirty million, and the exportation from Hamburg alone has been thirty million in a year. It is not surprising that so important a demand has raised the price of leeches till they have become a very profitable article of trade.
The successful stocking of a pond with leeches is a work requiring considerable care; the animal has many enemies, against which it must be protected, and will not thrive except under specially favorable conditions. The mother-leeches, when planted in the pond, lay their cocoons (which contain the eggs) as in nature, and the young brood is hatched out at the proper time; but this brood, besides protection, requires its natural food, sickens if it does not find it, and can not be fed artificially. The young worms will not thrive in artificial ponds; neither can they be transplanted from other countries and left to themselves without having first undergone a process of acclimatization. The most suitable ponds for acclimatizing leeches should be dug in bog-lands to a depth of about six feet, and should have from about six to ten inches of bog-soil on the bottom. The pond should contain about three feet of water, and should be provided with an inflow of fresh water and be surrounded with a wall two or three feet high. If the leeches are put in the pond in May or June, they will deposit their cocoons toward September in funnel-shaped holes in the peaty bottom; in the course of a few days some ten or fifteen young
Fig. 3.—8. The jaw, with its saw like tooth-plates. 9. The head, with its three-parted month opening. 10. The upper side of the head, with ten eyes. 11. Section of a cocoon with its eggs.
leeches will come out from the cocoon, and will attach themselves to the old one to suck from it till they are large enough to seek food for themselves. For food, the pond should be furnished with calamus and other reed-like plants; and duck-weed, little fishes, snails and frogs should be put into it. Toward the latter part of the fall the animals should be taken out of the propagating pond and put into a smaller pond with a solid bottom of clear loam or sand, from which they may be taken in the spring to the marshes and bogs, in which, from that time, they will increase quite rapidly.
The question whether leeches can be cultivated on a large scale with profit may be answered decisively in the affirmative by pointing to a few examples in which the business has been carried on successfully. The brothers Béchade hired a large swamp from Baron Pichon, near Bordeaux, as grassland, for a rent of three hundred francs; after stocking it with leeches they were able to have the rent gradually raised to 25,000 francs without feeling overcharged. Since they began their enterprise, in 1835, leech-culture has risen at Bordeaux to be a source of great profit, involving the application of 5,000 hectares (12,500 acres) of land to the purpose, employing a great many work-men, and representing a capital of several million francs. A land-owner in Mecklenburg is said to receive an income of not less than 18,000 marks ($4,284) from his share of the rent of a leech-farm. A physician at Liegingen, in Würtemberg, stocked a marsh of two and a half hectares (six and a quarter acres) with leeches in 1827, and succeeded so well with it that he was able to sell his worms by the hundred-weight.—Translated from Die Natur.
- Although the application of leeches has been diminished in consequence of the adoption of new practices in medicine, which permit bloodletting only in a limited degree, the use of the animal is still considerable, and always will be so. A few years ago, when bloodletting played an important part in sickness, leeches enough could not be got, and it was hard to satisfy the demand in the ordinary way. Five to six million leeches, costing a million and a half of francs, were used in the hospitals of Paris yearly from 1829 to 1836, and 187,000 pounds of blood were drawn annually, or 1,496,000 pounds in the eight years!