The Villi have been aptly compared to the Spongioles of the roots of plants, and seem to be endowed with the peculiar property of selecting the nutritive fluid and conveying it to the lacteal absorbents. These vessels do not reach to the extremity of the villi, nor do they open on the surface of the mucous membrane by any appreciable apertures; but the end of each villus is composed of a loose spongy tissue, in which a number of cells may be seen, in various stages of development, during the process of absorbing the chyle. It appears almost certain, from the recent observations of Mr. Goodsir, that the Absorption of chyle is really performed by these cells; and that a fresh crop (as it were) is being produced, every time that digestion takes place and chyle is prepared. In the interval, no cells can be seen in the ends of the lacteals; but they begin to be developed (from the germs that were left behind by the previous crop,) as soon as the chyle is prepared. Their growth is very rapid and their life is transitory. In growing, they absorb into themselves part of the fluid that surrounds them; and it is probable that, when they are mature, they either burst or dissolve away, and deliver this fluid to the absorbent vessels.

As we have already spoken of the Blood, it will be convenient now to notice the physical characters of the Chyle, and, with it, of the Lymph. Both of these fluids contain globules or floating Cells, and also fibrine; but they are more abundant in the former than in the latter. The chyle contains a large quantity of fat in a free state; while in the lymph and also in the blood, it is in a state of combination with other matters. The chyle, like the blood, contains iron, which it derives from the food; but the metal appears to be in a state of much less intimate combination in the former than in the latter fluid; for its presence can be much more easily detected by chemical re-agents.

The appearance and characters of the Chyle differ, in some important respects, according to the part of the lacteal system, from which it is obtained for examination. The following passage, from Dr. Carpenter's popular work, presents a good picture of these changes.

"If obtained," says he, "near the surface of the intestines, before it has passed through the mesenteric glands, it is entirely destitute of that power of spontaneously coagulating 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, seems 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 is evident that some of the albumen has undergone a transformation

the water penetrates and pervades the soil which lies between the intersecting streamlets, and thus reaches the growing plants, so the nutritious fluid, escaping through the coats of the blood-vessels, must permeate the intermediate mass of tissue which lies in the meshes of even the finest vascular network. The quantity of fluid supplied, and the distance it has to penetrate beyond the vessels, will vary according to the proportion which the latter bear to the mass requiring to be nourished."

into fibrine. At the same time, a great increase is observed in the number of certain floating cells, 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; these are colourless, and, like most other cells, contain smaller particles within them: their average diameter is about th of an inch. By the time that the chyle reaches the central receptacle (the throracic Duct) its power of coagulating has still farther increased; so that its resemblance to blood, except in regard to colour, is much stronger. The proportion of Fibrine and Albumen, which it contains, is much greater than that which existed in the first chyle, whilst the amount of oily matter is less; hence it seems probable that the latter has been partly transformed into albumen."

We shall not, at present, inquire what is the most probable cause of these successive changes in the constitution of the Chyle. Suffice it to say, that the accuracy of the preceding remarks has been amply confirmed by the most careful observations, and that the important and essential phenomenon in these changes is the gradually increasing number of the floating cells, as the Chyle approaches the thoracic duct.

In the Invertebrata, no lacteals nor lymphatics are discoverable in any part; the blood-vessels have therefore to perform the function of absorption. We have already stated that the blood of these animals is destitute of red globules, and has more resemblance to the chyle than to the blood of the Vertebrata.

Having thus briefly sketched the most remarkable features of the process of Absorption, we shall now, for a moment, turn to that of Secretion. As stated above, there is good reason to believe that the mucus of the intestines is secreted by the minute cells, which line the mucous membrane and the follicles dispersed upon its surface. These Epethelium-cells are continually coming forward, bursting and discharging their contents, are then replaced by a deeper-seated new crop, which advance to maturity, as the others pass away. The mucous follicles seem to serve the purpose of merely increasing the extent of the secreting surface, at those parts where a large amount of secretion is required.

and

But it is not the secretion of Mucus only that is effected in this way. All the other Secretions of the body appear to be performed in a similar manner, viz: by the agency of cells lining mucous surfaces; such surfaces being in the form either of tubes or of rounded follicles. All the conglomerate Glands are ultimately resolvable into an innumerable number of acini or bundles of follicles, which open into minute efferent ducts: by the union of these ducts the main excretory duct of the gland is formed. Such a gland, therefore, when freed from its blood-vessels and the cellular tissue which enters so largely into its composition, would resemble a bunch of grapes; each separate grape attached to its own little footstalk, and these gradually coalescing and forming the main stalk which upholds the bunch.

This description accurately represents the structure of the Liver in Crustaceous and Molluscous animals. In Insects, on the other hand, the hepatic organ consists of a few elongated tubes, and there is no vestige of any proper glandular or parenchymatous substance; while, in some of the Polypes, the only rudiments of a liver are a few mere follicles, lodged in the walls of the stomach, and which therefore are quite similar to the

intestinal follicles in the Mammalia.* These follicles,-whether simple as in the Zoophyte, or complex and conglomerate as in the higher animalsare believed to be invariably lined with Cells; which, by their constant formation and destruction, perform the important office of eliminating the particular secretion. We cannot indeed tell, nor even form a reasonable conjecture, why one set of cells should secrete Bile, another Mucus, a third Urine, and so forth. All that we can say is, that such is the fact; and that certain cells are endowed with the vital power of selecting from the circulating juices the particular elements of the secretion, which they are appointed to accomplish. The why is a mystery; but not a greater one than to explain how one portion of the petal of a flower should secrete a red dye, while the adjoining portion may secrete a yellow; or how some of the cells in the early embryo become converted into muscular substance, and others into cartilage or epidermis. Although much remains to be discovered, still it is an important point gained in our investigation of the process of Secretion, if we have ascertained with tolerable certainty that it is performed by the agency and interposition of cells.

A curious circumstance connected with this very important function is that, in certain states of the system, a secretion is sometimes eliminated, not by the usual apparatus destined for this purpose, but either by another gland or by another secreting surface. For example, the Urine has been known to pass off by the skin, or the bowels, when an obstruction has existed either to its secretion by the kidneys, or to its escape from the bladder; and the Milk has occasionally been discharged from the salivary glands, kidneys, &c. when something had occurred to prevent the mammæ from doing their duty.

This interchange, or vicarious performance, of function, will probably surprise the reader less, when he calls to mind the intimate analogy that exists, not only in many of the lower groups of animals, but also in the early stages of the development of the human fœtus,-between the cutaneous and mucous surfaces, and the very close resemblance in the mechanism, or elaborating apparatus, of the different secretions of the body. We have seen that there is good reason to believe that this invariably consists in the formation of the minute cells or vesicles upon an epithelial surface, and that every gland, however complex its structure may be in the higher tribes of animals, is resolvable into one or more simple follicles or tubes. This identity of structure, therefore, partly prepares us to anticipate that one gland may occasionally perform the duties of another. Until we know more of the Chemistry of life and the elective attractions of living organisms, we cannot expect to push our inquiries much further.

There still remain one or two tissues or component parts of an Animal body, to whose development and formation we wish to allude, as affording

* As another curious illustration of the fact, that form or outward structure of a gland has nothing to do with determining the nature of the secretion which it elaborates, we may mention that, while the urine is generally secreted in tubes —which are either closely conglomerated and associated together, as in the medullary substance of the kidney in the Mammalia; or are separated and detached from each other, as in Insects-yet in Molluscous animals, this secretion is effected by mere follicles.

additional illustrations of the great Physiological Doctrine we have been considering and first of the Fat or Adipose substance. This is to be regarded as a simple secretion that has been eliminated from the blood, and which is stored up in numerous sacs or cells, for the use of the individual organism. These cells are independent and distinct; they do not communicate with each other; and their function seems to be to separate from the circulating fluid the oleaginous particles, which constitute the Fat. The deposition, therefore, of this substance is strictly and truly a Secretion; and indeed a marked analogy may be traced between this process in animals, and the elaboration of the resinous and oily matter in the leaves of many plants; not only as regards the chemical proportions of the substances in question, but also because, in both instances, they are stored up in the cells which formed them, and then constitute part of the general fabric of the individual-not for the purpose of subsequent Excretion, but rather for that of self-Sustentation.

The pigment also, or colouring matter of the skin and choroid coat of the eye is found on examination to consist of numerous cells, which have the power of secreting a black granular matter in their interior.

The black-spots on the skin of the Frog, Water-newt, &c. are produced by the presence of similar pigment-cells; the colouring matter being secreted by and stored up in them, just as the fat is in the cells of the adipose tissue.*

If space permitted, we might now notice at some length the development and formative evolution of the Solid parts of the body; but this part of our subject we can only glance at. The structure of nascent Cartilage is always appealed to, as affording a good illustrative example of the Cellular Doctrine. With the aid of the microscope, it is readily perceived to be composed of numerous cellules lying near together in the midst of a quantity of an inter-cellular substance, that is very similar to what exists in the general tissue of many plants. The conversion of cartilage into bone is effected by the deposition within the cells of earthy matter, that has been eliminated from the blood.

The following observations on the subject, by Muller, may be aptly quoted here:

"The process of the development of Cartilage seems to be independent of blood-vessels, and to be wholly analogous to the process of growth in vegetable tissues. How the Canals, radiating from the corpuscles of ossified bone, are developed, is not known. Two hypotheses are proposed by Schwann. If the osseous corpuscles are the cavities of cells, the thickened

"The secreting cells not unfrequently possess the power of elaborating a peculiar colouring matter, either separately or along with the substances which seem more characteristic of the secretion. Thus the ink of the Cuttle-fish is in reality its urine, charged with a quantity of black matter formed in the pigmentcells that line its ink-bag; and the corresponding secretion in other Mollusca is rendered purple by the same cause. The bile seems to be universally tinged with a yellow colouring matter, which may be regarded therefore as an essential part of the secretion; in some herbivorous quadrupeds it has a green hue, and the colouring matter by which this is given, appears to be identical with that contained in the vegetable tissues on which they feed."-Carpenter.

walls of which have coalesced with each other and with the inter-cellular substance so as to form the mass of the cartilage of the bone, then the radiating canaliculi must be regarded as canals, extending from the cavities of the cells through their thickened walls, and would be analogous to the pore-like canals of some vegetable cells. But if the osseous corpuscles are the cells themselves, and not merely their cavities, (the whole substance between the corpuscles being intercellular tissue), in this case the canaliculi will probably be radiating prolongations of the cells extending into the inter-cellular substance. According to this latter view, which Schwann regards as the more probable, the canaliculi would correspond to the processes given off from some cells of plants."

The Nervous and Muscular tissues also are primarily evolved, in the early Embryo, as a mere agglomeration of cells. These cells by apposition and coalescence first form tubes; and then these tubes are subsequently filled with the substance peculiar to each, and become so very closely and intimately coherent that the tissues, when fully developed, exhibit no appearance of their original cellular structure. The ultimate reason for such a transformation is thus ingeniously propounded by the talented author of the Cyclopædia :

"It is easy to see that, so long as cells remain isolated from each other, they exist as so many distinct individuals-performing, it may be, the very same operations but doing this independently of one another. Now the very nature of the Animal (contrasted with the Organic-Rev.) functions requires that the actions of the several parts of the tissues, which perform them, should be the most intimately connected; thus, when an impression is made upon any part of the surface of the body, it has to be instantaneously communicated to the Brain; or an effort of the will, acting through the brain, has to call into immediate operation a large amount of muscular tissue. We could not conceive these functions to be performed by a number of isolated cells; and we find in fact that the muscular and nervous tissues are composed of tubes, containing substances that are peculiar to each respectively. These tubes originate like the ducts of plants, in cells laid together end to end, the partitions between which have broken down; and the deposits that are found within them, on which their peculiar properties seem to depend, are formed at a subsequent time."

IV. In the last division of our subject, we intended to point out the several applications that have been made of the Cell-doctrine to pathological inquiries, and to the illustration of various diseases. Neither space nor supply of materials permit us to enlarge at present upon this most interesting topic. All that we can do is simply to notice those morbid states and changes of the body which (it has been supposed) are produced by the presence of extraneous, so to speak, and abnormal cells, either in a part or throughout the system generally.

From the researches of that distinguished physiologist, Professor Muller, it appears that such growths as Cancer, Fungus Hæmatodes, Sarcoma, &c. are essentially cellular or vesicular in their structure; the morbid cells being produced by a transformation of the natural and normal cells of the part into others of a new and deleterious character. These morbid cells seem to be endowed with the same power of self-Reproduction, which we have shown to belong to the usual formative cells of all organised bodies; and indeed the very character of malignancy of the diseases in question is,