200 PROPERTIES OF CHYLE-SANGUIFICATION. through the glands, it is entirely destitute of that power of spontaneously coagulating, or clotting, which is so remarkable in blood: and when examined with a microscope, it is seen to present a number of oily globules of various sizes; together with an immense number of very minute particles or molecules, which also seem of a fatty nature; and to these last, whose diameter is between 1-24,000th and 1-36,000th of an inch, the milky whiteness which characterises chyle appears principally due. But the chyle drawn from the lacteals, after they have passed through the mesenteric glands, possesses the power of coagulating slightly; hence it would seem that some of its albumen has undergone a transformation into fibrin (§ 17). At the same time, a great increase is observed in the number of certain floating corpuscles, which are occasionally to be noticed in the first chyle, but which are very abundant in the fluid drawn from the glands and from the lacteals that have passed through them; of these, which bear a strong resemblance to the colourless corpuscles of the blood (§ 234), the average diameter is about 1-4,600th of an inch.By the time that the chyle reaches the central receptacle, its power of coagulating has still further increased; so that its resemblance to blood, except in regard to colour, is much stronger. The proportion of fibrin and albumen which it contains, is much greater than that which existed in the first chyle, whilst the amount of oily matter is less. 223. There can be little doubt that the change which the chyle undergoes in its passage through the lacteals, is partly due to the influence of the living walls of these vessels upon the fluid in contact with them, and partly to that of the colourless corpuscles which float in the fluid, and which form the principal constituents of the absorbent glands. The whole apparatus, indeed, may be looked upon as one great Assimilating Gland, having for its function to make blood out of crude nutriment; provided-for in the higher Vertebrata by the convolution of the lacteals in the mesenteric glands, and in the lower, by the simple extension of the vessels themselves. It is probable that, by being brought into very close neighbourhood with the blood in these glands, the chyle may be made to undergo some further change; although, as each fluid is contained in its own tubes, which do not communicate, there can be no proper intermixture. ASSIMILATING GLANDS- -ABSORPTION IN INVERTEBRATA. 201 224. There are certain glandular bodies, disposed in various parts of the system, which seem to discharge a similar office; withdrawing the raw material (so to speak) from the general current of the circulation, and returning it again in a state of higher elaboration. Such are the Spleen, the Thyroid and Thymus glands, and the Supra-Renal capsules. Besides these, the Liver probably exerts an assimilating action upon the crude materials which are made to pass through its substance, almost immediately after having been received into the blood-current, and before they are allowed to pass into the general circulation; the whole of the blood returned by the gastric and mesenteric veins from the walls of the alimentary canal, being conveyed through the liver by the portal system, in its way to the heart (§ 267). 225. In the Invertebrated animals, neither lacteals nor lymphatics exist; and the blood-vessels, whose absorbent powers are to a certain extent restricted in the higher animals, have to perform the functions of these. There are animals, however, which are destitute not only of lacteal and lymphatic vessels, but even of blood-vessels; and in these, as in the Cellular Plants, there is but little transmission of fluid from one part of the body to the other; for every portion, both of the internal surface (or lining of the stomach), and of the external surface which is bathed in the surrounding fluid (for most of these animals are aquatic), seems equally to possess the power of absorption; and the parts to whose nourishment the fluid thus received into the body is to be appropriated, are in the immediate neighbourhood of those which have absorbed it. This is the case, for example, in the Hydra and Sea-Anemone, and, more or less, in all the Polypes; as well as in the lower` Worms. Between these, therefore, and the Cellular Plants, a remarkable analogy exists in regard to the mode in which the nutriment is absorbed and applied; the difference being, that the Animal possesses a digestive cavity, lined by an inward extension of the external surface, which does not exist in Plants (§ 8). And it is upon the walls of this cavity, that the absorbent vessels of the higher Animals (whether lacteals or blood-vessels) are distributed, collecting the nourishment in contact with them; just as the roots of a Plant, spread through the soil, draw up that which it contains. But among those lowest animals in which the digestive cavity altogether 202 OF THE BLOOD, AND ITS CIRCULATION. disappears (§ 203), the function of absorption is not in any way limited; since every part seems to have the power of receiving from without, and of assimilating to its own substance, the nutrient materials which it needs. CHAPTER V. OF THE BLOOD, AND ITS CIRCULATION. 226. The processes that have been already explained, have for their object to prepare the nutritious fluid, which supplies the materials for the growth of the several parts of the body, and which is conveyed through them by the apparatus to be presently described. In Man and the higher animals, this fluid, which is known as the Blood, has a red colour, and contains a large quantity of solid matter. The redness of the blood has been mentioned as a distinctive character of the Vertebrated classes (§ 75); it exists in Mammalia, Birds, Reptiles, and Fishes, and in these alone. In the Molluscous classes, as also in most of the Articulated, the nutritious fluid is nearly colourless; and it will hereafter appear that this fluid bears, in some respects, a stronger resemblance to the chyle and lymph of the Vertebrata, than to their blood (§ 234). There is an apparent exception in the case of certain marine Worms, the fluid circulating in whose vessels has a reddish hue; this does not depend, however, upon the presence of any red par ticles, such as are characteristic of the blood of Vertebrata (§ 229), but upon a reddish tinge in the fluid itself, which does not seem altogether to answer to the character of blood (§ 294). 227. The blood of all the higher animals exists in two different states. When it is drawn from a slight scratch or other wound of the skin, it is of a bright red hue; whilst that which is drawn in bleeding from the arm, is of a dark purple. The former is termed arterial blood, because it is contained, for the most part, in the tubes which are called Arteries, and which are conveying it from the heart to the tissues it has to nourish. The latter is called venous blood, because it is drawn from the Veins, by which it is returned from the tissues to the heart, after having performed its part in them. Hence it VENOUS AND ARTERIAL BLOOD. 203 is evident that this change of character has been produced during the passage of the blood through the tissues; and so important is the alteration, that the blood which has been subjected to it is not fit to pass again into the arteries of the body, until it has been renewed by exposure to air in the Lungs. In their vessels, the contrary change-of which the nature will be presently explained (§ 253)-is effected, the dark hue of venous blood giving place to the bright red of the arterial fluid; this is again changed during the passage of the blood through the body, to be again restored in the lungs. The same is the case in regard to Fishes, whose gills perform the same function as the lungs of air-breathing Vertebrata. And among the Invertebrated classes, although the deterioration of the blood in its passage through the body is not made manifest by any change of colour, yet its renewal by exposure to air in the respiratory organs is not less requisite. 228. Hence the continual movement of the blood is necessary for two purposes in particular;-first, to convey the nutritive materials from the place where they are received and prepared, to that in which they are appropriated, and thus to afford to every organ a constant supply of the materials which it requires; and, second, to carry this fluid, at regular intervals, to certain organs by whose instrumentality it may be exposed to the influence of the air, so as to regain the qualities it has lost, and part with what it has taken-up to its prejudice. But there are many other objects fulfilled by it, which will unfold themselves as we proceed. Properties of the Blood. 229. When the circulating blood of a red-blooded animal is examined with a microscope, it is seen to consist of two distinct parts;—a clear and nearly colourless fluid, to which the name of liquor sanguinis (or liquor of the blood) is given; and of an immense number of rounded particles floating in this fluid, which are often termed the globules of the blood. The shape and size of these particles are, for the most part, very uniform in animals of the same species; but in no instance are they globular; and it is better, therefore, to term them corpuscles. In Man and most other MAMMALS, they are nearly flat discs, resembling pieces of money, but usually exhibiting a slight depression towards the centre (fig, 115). 204 BLOOD-DISCS OF MAN AND MAMMALS. No nucleus can be distinguished in them, but they present a dark central spot, which is an optical effect of their bi-concave form; and this spot may be made to disappear by the addition Fig. 115.-RED CORPUSCLES OF HUMAN BLOOD. Seen separately at A, a a showing the front view, b the profile or edge view, and * three-quarter view; at B united with each other so as to form columns like piles of money; at c in a state of alteration such as exposure to air will produce; D shows a colourless corpuscle, or lymph-globule. a of water to the liquid in which they are suspended, the discs first becoming flat, then bulging-out on either side, and at last swelling so as to burst. The reason of this will be presently explained (§ 231). In MAN and MAMMALS generally, the diameter of these blood-discs varies from about 1-2800th to 1-4000th of an inch; but in the small Musk-deer, it is less than 1-12,000th. In the Camel tribe, the discs are oval, as in the lower Vertebrata. 230. In Birds, Reptiles, and Fishes, the blood-particles present some curious differences from those of Mammalia. In the first place, they are much larger; their form, also, is oval instead of being round; and instead of being depressed in the centre, they bulge-out on each side. This bulging is Fig. 116.-BLOOD CORPUSCLES OF PIGEON. At A are seen the red corpuscles a, b, and the colourless, or lymph globules c, c; at n, a red corpuscle treated with acetic acid; and at c, the same treated with water, so as to render the nucleus more distinct. evidently occasioned by the presence of a nucleus which is more solid than the rest; the nucleus, however, is not so well |