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THE LUNGS, HEART, AND BLOOD-VESSELS OF
THE SLUG.

BY HENRY LAWSON, M.D.,

Professor of Physiology in Queen's College, Birmingham.
(With a Tinted Plate.)

We all know that man has lungs and a heart, and that the blood circulates in his body, but there are many of us who would be very much astonished to learn that as low an animal as the slug has a heart and lungs also, and that its blood circulates through arteries and veins, and is propelled through the former by the pulsations of the heart.

I hope to show in the present article that the mollusk in question has all the organs to which I have alluded, and to explain to you the steps by which you may arrive at the knowledge on your own account, should you be inclined to exhibit scepticism regarding my statements and assertions. It is very interesting to be informed that one creature possesses one gland, and that another is known to have another, but it is vastly more satisfactory to be able to take one's scalpel and forceps in hand, and demonstrate for one's self the accuracy or it may be the falsity of an author's statements. For although an anatomist, well known for his original researches, may pledge himself to some particular assertion, it by no means follows as a logical conclusion that that assertion is true. The most skilled in observing natural objects may be occasionally deceived, or may be so blinded, owing to some foregone deduction, as to see only those things which they had anticipated, to the exclusion of others as important but which do not touch immediately on their darling speculations. Hence, as I have just mentioned, I propose pointing out to you the mode which you must pursue in order to satisfy yourself regarding the truthfulness of the observations which I have here committed to paper.

We have to study two great systems in the economy of the slug the respiratory and circulatory; and as facts are of very little value unless we associate them with ideas, let us ask what is the use of a respiratory system? The object of a lung is to expose the blood freely to the influence of the air. The function consists in certain operations going on simultaneously in the blood and atmosphere, and which result in the purification of the former.

The blood has two important duties to discharge.

1st. It absorbs from the stomach the valuable portions of the food, which it carries to the different tissues in order to repair them.

2nd. It abstracts from the tissues the refuse matter of the

old structures, and carries it away to the various sewers (glands, etc.), of which the lung is one.

This refuse or effete material is of many sorts, each of which is removed by a separate channel, thus :-Urea is got rid of through the kidneys, bile through the liver,* and lactic acid through the skin. In the lung, the foul air (carbonic acid) is discharged, the oxygen of the fresh air is taken in, and the blood is altered in properties and constitution.

Having your slug fixed in the way described in a former number,† seek the pulmonary opening. This is placed on the right side, in the middle lateral line, and at about half an inch from the right upper tentacle. It is of an elliptical, or rather of a double wedge-shaped outline, and may readily be distinguished whilst the animal is breathing, owing to its snowwhite lining, which is now and then everted, contrasting markedly with the dark black hue of the outer skin. Before you begin your dissection, it will be as well to map out distinctly the extent of the lungs, which may easily be done as follows;-Take a thin glass tube, slightly drawn out at one end, and insert it into the orifice, which, with a little care, you may easily do; then blow the air somewhat forcibly from your mouth, and the lungs of both sides will become visibly inflated, together with the passages of intercommunication, in this manner indicating with clearness the exact limits of the pulmonary system.

Having formed a general notion of the whereabouts of the lung sacs, next place the point of your curved knife within the orifice, and keeping the blade as horizontal as possible, cut steadily backwards, and in a line parallel with the middle lateral plane till you reach the membranous constriction which separates the thorax from the abdomen; at this partition the lung ends posteriorly. As yet you have only exposed the right lung, but since it communicates with its fellow of the opposite side through two distinct channels, by laying them bare, you may then come upon the left sac.

Place your long-bladed scissors in the hinder passage (which lies quite in front of the constriction referred to, and in the transverse plane), and with the blades as close to the partition as possible, cut from right to left through the integument. You now reach the left lung sac, but before you can examine it you must make two other incisions.

1st. Cut from behind forwards, parallel to your first incision, till you arrive at the anterior end of the sac.

*The bile is not entirely worn-out matter, for although if retained in the system (of man), it tinges the skin and poisons the brain, producing jaundice, it is in some unknown manner subservient to the ends of digestion.

+ Vide INTELLECTUAL OBSERVER, for May, 1863.

2nd. Cut transversely from left to right through the front channel of communication, and parallel to your second incision, till you reach the remnant of the pulmonic aperture.

By these four incisions you have laid open the lungs, which present themselves, not as a ring,* surrounding the undivided central portion (the heart and shell-bag), but as two distinct pouches of oblong form. These pouches enclose between them the heart, heart-gland, and the shell-bag, and are shut off below, behind, and in front, by folds of thin lining membrane, which pass from the inner surface of the general skin of the body. Each division measures about half an inch in length, and is somewhat more than a quarter of an inch deep; the width is inconstant, depending, as it does, upon the condition of the body as to elongation or contraction. The walls of the lungs are composed of the general skin, which has within it a snow-white lining. This latter is the true respiratory surface. Blood-vessels cannot be said to ramify in it, but it is literally tunnelled by passages which interlace in the most intricate manner, forming a network in which the blood is exposed to the influence of the atmospheric air, introduced through the lung-opening. The blood which circulates in the lungs is not sent there directly from the heart (as is the case in man), but is carried to these sacs by two great veins, one of which lies on each side of the body, and, by the aid of numerous branches which contribute to form it, conveys the vital fluid from the abdominal cavity to the pulmonary networks. The course which the blood takes in traversing the lungs we shall investigate under the head of

ORGANS OF CIRCULATION.

In vertebrated animals the vessels through which the blood travels, in "going its rounds," are of three kinds :

1st. Thick-coated vessels, which bring the blood from the heart.

2nd. Microscopic channels called capillaries,† which are present in almost every tissue of the body.

3rd. Thin-coated tubes with valves, which carry the blood from the capillaries to the heart; these are called veins.

Formerly, when the habit of drawing analogies between vertebrates and invertebrates was more frequent than it is in our day, it was supposed that in all gastropods what was termed a complete circulation existed; that is to say, the blood

Von Siebold and others have stated that the lungs in this creature are of an annular character. I leave the reader to judge between myself and so learned an anatomist.

+ So called from "capilla," the Latin for a hair, but they are very much smaller than any hair, and clusters of them placed together would not be as thick

as a human one.

passed from the heart into arteries, from these into capillaries, and from the latter into the veins, and thus returned to the heart again. This view, as we shall see presently, was erroneous. It was at first held most persistently by Cuvier, who, although he had demonstrated the absence of capillaries in the genus Aplysia (sea-hare), contended, nevertheless, that this was but an exceptional instance, and that for the most part head-bearing mollusks had a complete circulatory system. In this century the most formidable controversy upon the ques tion has taken place between M. De Quatrefages and M. Souleyet; the former maintaining, correctly enough, that in these beings the circulation is not perfect, and the latter asserting with equal determination that it is perfect.t

I have paid no inconsiderable attention to this subject, and I believe I have succeeded in showing the course of the circulation, and also the absence of capillary vessels. The blood having been expelled from the heart (Fig. 1), passes through the first great artery (aorta), and from it through its various divisions and sub-divisions till it reaches the stomach, intestines, head, liver, etc., etc., and at last arrives at the ends of the arterial vessels ;§ and as these are quite open, the fluid escapes from them. What then becomes of it? It flows into the great abdominal cavity-that sac formed by the integument in which are placed the important organs, which constitute the great bulk of the animal. Here it bathes the digestive tube and the great glands, and sooner or later is admitted into the veins, and travels to the lungs. The general veins, like those of the lungs, are channels grooved in the skin, and are not distinct vessels, such as the arteries. They ramify in the abdominal integument, springing on either side from a large lateral branch which commences at the tail and terminates in the lung, and by which all the blood that had been thrown out from the arterial tubes is re-collected and carried to the respiratory surface.

At this point difficulty has been invariably experienced in ascertaining the precise direction in which the stream of blood flowed-one side contending that of the entire current, a portion passed to the so-called kidney, while the remainder flowed through a special pulmonary vessel to the heart; the other, that the blood was poured at once into a sinus or lacuna. Both these ideas I believe to be incorrect, the more so as I

* Règne Animal, "Mollusques,” p. 50:

Comp. Rend. xix. and xx.

Quarterly Journal of Microscopical Science, January, 1863.

§ Professor Milne Edwards says, that in most gastropods the aorta ends in a sinus (or cavity) containing the brain, gullet, and salivary glands, vide Frorieps Neue Notizen, xxxiv. pp. 80, 260. This is not the case in the slug.

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