parent vessels, and of small oblong bodies called glands. The vessels are plentifully furnished with valves, which gives them a knotted appearance. They arise from all the internal cavities, from every part of the surface, and from every organ of the body. In their course, they invariably pass through one or more of the absorbent glands; and, uniting into larger and larger branches, they at length form one common trunk called the thoracic duct; which pours its contents into a large vein, near the heart, and there mingles them with the general mass of blood. The absorbent glands consist of a number of small cells enveloped in a membranous covering, in which is deposited a somewhat viscid fluid. They are found to be most numerous in the higher classes of animals; but in many of the lower classes they are totally wanting. The orifices of all the absorbent vessels, both those which suck up the chyle from the alimentary cavity, and are termed lacteals, and those which imbibe substances applied to the skin, and are called lymphatics, commence in what are denominated ampullula, which are small oval vesicles, composed internally, like the absorbent glands, of minute cells, and containing within them a fluid of a viscid nature. The function of absorption is not confined to the lacteals and to the lymphatic vessels. Majendie has shown that it is likewise performed by the veins. That the lymphatics and the veins should exercise the same function is not remarkable; for they both possess the same structure; both arise in the same manner; both are furnished with valves; both carry their fluids from branches to trunks; both absorb their contents on the same principle; and both circulate them by the agency of the same power. The lymphatics, in fact, are nothing more than a subordinate system of veins.

The process of absorption is carried on in plants as extensively as it is in animals: it is performed, too, by similar organs, and on a similar principle. The roots take in the nutritive fluid of the soil in which they are placed, and moisture is plentifully absorbed by the stem and branches. When we examine a root, we find that all the delicate fibrils growing out of it are terminated by small oval-shaped bodies called spongioles. These spongioles resemble, in their structure and their form, the ampullulæ and the glands attached to the absorbent system of the animal body. Like them, they are composed of small cells, enclosed within a membrane, containing a viscid fluid. They likewise receive the open extremities of the capillary tubes, of which the radical fibrils principally consist, precisely in the same manner as the ampullulæ receive the orifices of the lacteal and the lymphatic vessels.

The vegetable lymphatics, or veins, which arise from the spongioles, are extremely fine tubes. They ascend through the stem, and convey the nutritious materials from the roots to the leaves, where they terminate. In many instances they pass through glands; as, for example, in the gramineous plants, the stems of which have placed upon them, at certain spaces, knots or joints; which, in their structure, are very analogous to the absorbent glands of the animal system. It has long been a subject of enquiry among physiologists, to ascertain upon what principle the contents of the absorbents effect their entrance into these vessels. Some have supposed that they enter on the principle of capillary attraction; others, that they enter by the operation of the vital power: some, by filtration, or imbibition through the coats of the vessels; and others, again, by the pressure of the atmospheric air. Dutrochet, however, ascribes absorption to a new principle, which he calls endosmose. It is difficult to decide which of these theories is the most correct. That of Dutrochet appears to me to be the least objectionable. I may remark, however, that, upon whatever principle we explain the entrance of substances into the mouths of the lacteals, the lymphatics, and the veins, belonging to the animal body; on the same we can account for the ingress of substances into the orifices of the absorbents and veins of plants.

From absorption, we pass on to the function of circulation. As soon as the chyle, the decayed corpuscles of the system, and the various substances brought in contact with the surface, have entered within the orifices of the absorbents and veins, they are carried along the minute ramifications of these vessels into their larger branches. At length they are all poured, by one or two large trunks, into a vein situate at the left side of the neck. In this vein they are mixed with the whole mass of blood, and conveyed along with it, to the right side of the heart. The venous blood, now replenished with new materials, is propelled from the right cavity of the heart, into the pul monary artery. By the branches of this vessel it is conveyed through the lungs, where it is exposed to the vivifying influence of the atmospheric air. After it has been duly elaborated in the lungs, it is returned, by appropriate vessels, to the left side of the heart, the contractile power of which forces it into a large artery called the aorta; whose numerous ramifications convey it to every part of the body, furnishing materials for growth, for nutrition, and for the supply of all the various secretions. After serving these purposes, the blood is deprived of its nutritive properties, and, being mixed with the worn out particles, it becomes deteriorated, and unable to

afford support or nourishment; it is, therefore, received by the extremities of the veins, and brought back by them to the heart, to acquire the addition of fresh materials from the food, to be again ventilated in the lungs, and again to be distributed all over the body. Such is the circulation of the blood, as it is exhibited in man and other animals of the Mammalia class, in which the heart is of the most complex character, and consists of four distinct cavities. In Fishes, the construction of the heart is much more simple, and possesses only two cavities. As we descend in the scale of animal life, we find the heart presenting a progressively greater simplicity of organisation; for, in the more perfect order of Vérmes, the only appearance of a heart that can be seen is one or two small dilatations appearing on the branches which unite the abdominal and dorsal blood vessels together. As we descend another step, we discover that there are some animals, destitute of a heart altogether. This is the case with some of the genera of the Mollusca, and the higher order of Zoophytes. In these animals, the circulation is carried on by only two distinct sets of vessels. By one set, the blood is distributed all over the body; by the other, it is brought back to the point, whence it first set out. In animals still less perfect than Zoophytes, as the Infusòria, and the inhabitants of sponges, there is no circulating apparatus whatever to be found.

In the higher order of plants, the sap, like the blood of animals, to which it may be considered analogous, is perpetually flowing in a complete circle. We perceive it carried up by the vegetable lymphatics or veins, from the roots, along the stem and branches, to the leaves. In the upper surface of each leaf, it is circulated through thousands of minute vessels; where it is acted on by the air and light. As soon as it has undergone its proper changes, it is received by the capillary branches of another set of vessels, which are ramified upon the lower surface of the leaf. These reconduct it along the branches and stem, towards the root. In its progress downwards, it supplies abun dant matter for the nourishment and increase of the plant, and the expenditure of all the different secretions. What remains after all these purposes have been answered returns to, and oozes through, the spongioles; to be again absorbed along with the other substances from the soil, to be again circulated through the stem and branches, and again subjected to a repetition of all the same processes. This description will be sufficient to show the features of resemblance between the course of the sap in vegetables, and that of the blood in animals.

In the vegetable system, we see the crude sap, or lymph, conveyed from the root to the leaves, in the same manner as

we see the imperfect blood, or chyle, carried from the stomach to the lungs; we see the sap elaborated in the leaves by the air, as the blood is elaborated by the same agent in the lungs ; and we observe the sap, after being rendered, during its progression through the leaves, fit for the support of the vegetable functions, distributed all over the plant, just as the blood, after its ventilation in the lungs, is conveyed to every part of the body. I do not mean to say that vegetables possess a circulating apparatus as complete as that of the higher orders of animals. They have no heart, or central organ of circulation; and the vessels which return the elaborated sap from the leaves are, according to Dutrochet, not continuous tubes, like arteries, but consist of small cells or sacs, called clostres, which are joined together in such a manner as to resemble a chain of beads. We have seen, however, that, if the heart is wanting in plants, it is likewise wanting in many of the lower classes of animal beings; and it is perhaps to the circulation of blood in these, that the circulation of the sap ought to be compared, although, in many respects, the latter is more perfect and exact than the former. I have already observed that there are some animals which possess no organs of circulation whatever; the same deficiency occurs in some plants, as the mosses and ferns. In these, the sap passes from one cell to another, in the same manner as the nutritive fluid permeates the gelatinous substance of the polypi and other animals, in which there exists no vascular system.

Ever since the discovery of the circulation of the blood, physiologists have endeavoured to ascertain the efficient causes by which this function is carried on. The mechanical physiologists imagine that the heart is the sole agent in the circulation of the blood; that the blood vessels are in no way concerned in the operation; that they are mere tubes, and that the blood is propelled from the arteries into the veins, and from the latter into the right auricle, by the contractile power of the heart alone. Another set of physiologists consider this opinion as not correct; and affirm that the circulation of the blood is not effected by the action of the heart alone, but likewise, in some degree, by that of the arteries, all of which they suppose to be more or less contractile. They allow that the larger arteries have but little contractility, and the blood which is transmitted through them receives its chief impulse from the heart. The capillary or small arteries, however, they regard as possessing a considerable share of contractile power, by the agency of which the blood is projected along their ramifications into the extremities of the veins. They consider that the veins are little more than elastic tubes; that their action is

entirely mechanical and that the progression of the blood throcen them is promoted by the visa tergo, [propulsion from behind), and by the pressure of the muscles during their contraction. Ancrg the individuals who hold that the arteries exhiba no manifestations of contractility, are Haller, Bichat, Craigie. Nysten, Pang, and Doilinger; while Hunter, Whytt, Senac, Thompson, Philip, Bikker, and Rossi are among those who entertain a contrary opinion. Wilson supposes that the force of the heart projects the blood through the arteries, and even into the veins, but that it is impelled along the venous trucks into the heart by what he terms derivation. Wilson's theory has, with considerable modifications, been adopted by Carson.

Dutrochet, a physiologist of great eminence, conceives that every hypothesis hitherto offered to explain the circulation of the blood is inadequate to the purpose. He has therefore proposed a new one. He seems, indeed, to allow that the power of the heart transmits the blood through the large arterial trunks; and, when there are no capillary branches between the arteries and veins, that this power extends from the former into the latter, and alone propels their contents forwards. He maintains, however, that the progression of the fluids through the capillaries is perfectly independent of the impulse of the heart; at the same time, he denies that these vessels circulate their fluids by the agency of contractility, because this is a faculty, he asserts, which they have never yet been proved to possess. Neither does he admit that the pressure of the contracting muscles is the chief cause of the motion of the blood through the veins and lymphatics. Dutrochet is of opinion that the circulation of the blood is conducted by the operation of endosmose [impulse inward]. I shall first endeavour to explain how this principle carries on the circulation of fluid through the lacteals. The extremities of these vessels, as I have before observed, commence in what are called ampullulæ, which are composed of small cells, filled with a dense organic fluid; a condition peculiarly adapted for the exercise of endosmose. In consequence of their endosmosmic power, the ampullulæ of the lacteals cause the chyle, by which they are surrounded, to flow towards them, and to enter their cells. This ingress produces an accumulation of chyle within these organs, which renders them very turgid. In consequence of this turgidity, their natural elastic power is called into play, which, reacting upon the accumulating chyle, forces it into the lacteal tubes. By the introduction of fresh chyle into the cells of the ampullulæ, the current is unceasingly kept up. The lacteal tubes are furnished with glands, which possess the same structure,