mencé, according to Lieberkühn and other microscopical observers, by patulous orifices on the surface of the villi. The latter projections, so named from a comparison with the pill of velvet, are very numerous on the circular projecting folds of the internal coat, called valvulæ conniventes. These latter not only render the progress of the chyle slower, but increase very greatly the absorbing surface, and penetrate, in consequence of the intestinal contractions, into the midst of the chyle in quest of its nutritive particles. A fluid is secreted from the intestine, analogous to that furnished by the stomach, although an accurate examination of its nature and properties is one of the physiological desiderata, (succus intestinalis). At the same time that the absorption of the chyle from the villous surface of the intestine is going on, it is moved gradually downwards towards the large intestine by the peristaltic motion. This is an undulatory and gentle constriction, taking place in several parts of the tube at once, and producing therefore a singular appearance, compared to the crawling of worms, and hence termed vermicular. It moves the chyle repeatedly over the surface of the intestine; and though it must urge that fluid partially upwards, yet its chief effect in the healthy state is exerted in the opposite direction. It is chiefly in disease that an antiperistaltic movement occurs, which conveys bile into the stomach, and even the whole contents of the small and large intes tine.

By the powers now explained, which propel the alimentary mass by their contractions, and by the admixture of the various fluid menstrua which dilute and alter its properties, those memorable changes are effected by which our food is said to be animalized or assimilated. In the duodenum and upper part of the jejunum it forms an equably mixed fluid, of the consistence of thick cream, greyish, and rather acid. Lower down it separates into two parts; the excrementitions of a pale brown or yellow colour, and nauseating smell; and the true chyle, separated from the former by the bile, and swimming on its surface.

Action of the large Intestine. The excrementitious portion of the chyme, deprived almost entirely of every nutritive portion, enters the cæcum: its passage through the last part of the ileum being favoured by a copious secretion of mucous from the glands which abound in that part. The valvula coli, which is the boundary between the

large and small intestines, is designed to prevent the contents of the former from regurgitating into the latter: and it performs this office in general; for nutritive clysters would otherwise enter the small intestines, and thus enable us to administer food enough for the support of life per anum: yet it occasionally fails in its office: hence vomiting of feces, and of tobacco clysters.

The large intestine may be regarded as the organ in which the residue of the chyme undergoes its last change; viz. the conversion into feces; as a reservoir, protecting us from the disgusting necessity of having that residue constantly flowing off; and as the instrument of its final expulsion from the body.

Some absorbents exist in this last part of the alimentary tube, and convey what nutritive parts still remain in the intestinal contents; but they are comparatively few, and hence the impossibility of nourishing the body by way of clyster.

The conversion of the alimentary residue into feces is owing to some active operation of the intestine; as these are very different from the mere putrid remains of animal or vegetable matter. And when this operation is deficient, portions of the food are seen in the evacuations per anum but slightly changed. The fecal matter is conveyed onwards by the peristaltic motion of the large intestine: it becomes thicker and more consistent in its progress, and is usually formed more or less decidedly by the cells of the colon. Its natural consistence is just sufficient to retain these marks; and its appearance throughout should be homogeneous. The colour depends on the admixture of bile, and in the most natural state is of a yellow brown. Although the change which reduces the residue of our food to feces is so far from being mere putrefaction, that the excrement is in fact less prone to putrefy than other animal matter; yet there is, as in putrefaction, a disengage. ment of gaseous products, particularly of sulphurated hydrogen. The colouring matter of vegetables is often seen in the feces, as the green of spinach, and red of beet root; and the fibrous indigestible parts of vegetables, as the skin of fruits, husk of grain, &c. The latter, indeed, is so little affected by the powers of digestion, that when the covering is entire, a grain may pass through the body, and still retain the power to germinate.

The expulsion of the feces takes place when they arrive in the rectum, which is

speedily irritated by their presence; and is performed partly by the muscular coat of that intestine, and partly by the muscles of respiration, producing the effort called straining. The periods of voiding the excrement vary, from several causes: they are more frequent in the young subjects, where the stools are more liquid. In the adult they should not be less, in a healthy state, than once in twenty-four hours.

Urinary Secretion. The liquids which we drink, absorbed by the lacteals, together with the nutritive part of the solid aliment, dilute the latter, and serve as a vehicle for it. They increase the quantity of the blood, and render it more fluid; conveyed into every part of the circulating system, they penetrate all our organs, carry away the particles detached from them in the different vital processes, and are then separated from the mass of fluids by the urinary organs, together with various other substances, whose retention in the body would produce injurious effects. The kidneys, therefore, dispose of the residual part of our liquid ingestu, as the feces are formed by the more solid foods, and the quantity of urine may, of course, be expected to vary according to that of the drink. All the old parts of the frame, which are constantly removed by the absorbents, while new depositions are formed by the nutrient arteries, go off in the same way; and hence the urine, although apparently a watery fluid, and called in common language water, contains a great deal of animal matter.

From the above account it will be readily understood, that the properties of the urine must vary according to the time at which it is voided after meals; the quantity of food and drink, the age and complaints of individuals, &c. Physiologists have distinguished urine of the drink, chyle, and blood. The first is a watery fluid, almost colourless, evacuated very soon after drinking, and possessing very slight urinous characters; the second, evacuated two or three hours after meals, is better elaborated, but not yet complete in its constituent principles; the last, voided after the repose of the hight, has all the properties of urine in an eminent degree. In infants it possesses no phosphate of lime nor phosphoric acid, as those substances are employed in the business of ossification, which is then active. In old persons, on the contrary, where the bony system, already overcharged with phosphate of lime, refuses to admit any more, this substance is carried off by the

kidneys. It is removed in the same way in rachitis and mollities ossium, where the bones become softened by its absence.

The great quantity of saline and crystallizable elements contained in the urine, account for the frequency of calculous concretions, which are found by recent and accurate analysis to vary very considerably in their composition. As there is no substance in the body which may not be evacuated by the urine, and manifest itself in that liquid, se, under different circumstances, every thing possessing a power of concretion may become the subject of urinary calculus. This diversity of constituent elements, together with the want of characteristic symptoms of the different species; and the irritation which the coats of the bladder must experience from chemical reagents, will convince us how extremely difficult, if not impossible, it must be to discover a lithontriptic that would obviate the necessity of a surgical operation.

The urine is very speedily and sensibly affected by certain substances; thus asparagus occasions a remarkable fœtor in this fluid: and turpentine imparts to it a violet odour. For a further account of its composition and physiology see the article URINE.

Almost every physiologist has noticed the rapidity with which this secretion is carried on: aqueous fluids, taken by the mouth, are sometimes separated so quickly by the kidneys, that an immediate communication has been suspected between the stomach and kidneys, on the supposition that there had not been a sufficient time for the fluid to arrive at the latter organ in the regular course of absorption and circulation. This conjecture derives no countenance from anatomy, and the size of the renal vessels explains the fact without any such supposition.

Absorption, or the process by which the chyle, separated from the food by the digestive organs, is carried into the blood, naturally follows the account of digestion. We have very little to add to what is stated on this subject in the article ANATOMY.

The admission of matter into the orifices of the absorbing vessels has been accounted for in various ways. Some physiologists consider it as a case of capillary attraction. But a little reflection is sufficient to shew that the absorbents are not like capillary tubes immersed in a fluid. Besides, were

such attraction the cause of absorption, that process should be carried on with regularity. On the contrary, absorption is occasionally very deficient, when abundance of fluids is presented to the mouths of the vessels, as in oedema; and in other cases, after being for a long time inactive, it is suddenly exerted to a great extent; thus large abscesses have been dispersed in one night. Others have endeavoured to discover some propelling power which should protrude the matter subject to absorption into the mouths of the vessels. The pressure of the atmosphere on the surface of the body has been considered adequate to this effect, and the deposition of new matter by the secerning artery has been assigned as the cause of the propulsion of the old particles into the orifice of the absorbent. On this theory, secretion and absorption should correspond more exactly than they are known to do. Mr. J. Hunter acknowledged that he was unable to account for the ef fects produced, unless by attributing to the mouths of the absorbing vessels powers similar to those which a caterpillar exerts when feeding on a leaf.

Some suppose that the absorbents cannot take up any matter that is not fluid; consequently that animal solids must be converted into fluids before they can become fit subjects for absorption; and that probably some solvent fluid is secreted for this purpose. The latter fact rests on no direct proof, and the whole hypothesis is very unlike the simplicity observable in other parts of the animal economy. It seems better, in these difficult investigations, to note facts than to form theories; and whoever contemplates the things done in the animal body, will be astonished at the power of the vessels, by whose agency they must be effected; a whole bone may perish, as, for example, that of the thigh, and may be increased by a new one; the vascular lining of the new bone will altogether remove the dead one.

Besides the great and leading office of the absorbents in conveying the chyle into the venous system, their agency is discerned in various other parts of the animal economy. The nearly transparent fluid that lubricates the interstices of the cellular substance, and the serous exhalation poured into circumscribed cavities, are taken up by the lymphatics, which must commence in all parts of the body by open orifices. When the due balance does not exist be tween the absorbing and secreting vessels,

the cellular substance becomes loaded with fluid (anasarca), or circumscribed cavities are rendered dropsical. Together with the lymph or fluid which the absorbing vessels derive from the sources just-mentioned, they convey from every part of the body the old constituent materials of our organs in proportion as new particles are deposited by the arteries; and these different elements are intimately mingled and combined in their passage through the absorbent glands, and the plexures of lymphatic vessels.

It has been a disputed point whether absorption goes on from the surface of the skin, while the cuticle is entire; the arguments on the affirmative sides are an alleged increase of weight in the body after a walk in damp weather; the abundant secretion of urine after remaining for some time in a bath; the evident swelling of the inguinal glands after a long immersion of the lower extremities in warm water; the effects of mercury administered by friction, fumigation, &c. It has been stated in opposition, that oil of turpentine has not been absorbed after long immersion of the arm; that solutions of medicated substances have not been taken up under similar circumstances, &c. We think it is sufficiently proved, that absorption from the surface does take place in the human body, but whether this extends, as a modern physiologist has stated, to gaseous bodies, cannot yet be decided.

It appears probable that the internal surface of the bronchi and pulmonary vesicles is an absorbing surface. For when a person breathes air loaded with the vapour of turpentine, that substance very speedily shews itself in the urine, although the skin will not take it up. If the body really increase in weight in damp air, it might be accounted for by means of pulmonary absorption. It must probably be rather in this way, or by the skin, that contagious matters affect the constitution.

The absorbents are concerned in producing changes in the different secreted fluids: they remove the aqueous portions of the bile and urine, and often take up even the colouring parts of the former, and convey them into the blood, whence they are deposited in all our organs, and produce the yellowness of jaundice. They introduce various diseases into the human frame, as syphilis, hydrophobia, inoculated small-pox, &c.; and in other instances act in a curative manner by taking up extravasated blood, by reducing swollen parts, &c.

The circulation, is the motion by which the blood, setting out from the heart, is constantly carried to all parts of the body in the arteries, and returns to the same point in the veins. The uses of this circulatory motion are, to submit the blood altered by the mixture of lymph and chyle to the contact of the atmosphere in the lungs (respiration), to convey it to several organs in which various animal fluids are separated from it, (secretion); and to every part of the body, for supplying its growth and repairing its losses, by means of its nutritive particles when completely assimilated (nutrition). The conveyance or transport of our fluids, rather than their elaboration, is the office of the organs of circulation. In this view they may be compared to those labourers, who in a large manufactory, from which various products issue, carry the materials to the workmen employed in the actual fabrication. As among the latter there are some who purify and bring to perfection the materials furnished by others, so the lungs and secretory glands are constantly employed in separating from the blood all those heterogenous matters which could not be assimilated to the substance of our organs.

The word circulation, when used absolutely, comprehends the whole course of the blood, as well in the lungs, as in the arteries and veins of the body at large. The greater circulation is the passage of the blood from the left side of the heart, through the arteries, to the extremities of the body, and its return through the veins to the right side of the same viscus. The lesser circulation is the transmission of the blood from the right to the left side of the heart, through the lungs.

The course, which the blood takes, has been already explained in the article ANATOMY. We subjoin the proofs and experiments, by which the facts there stated are supported.

The passage of the blood through the heart, i. e. from the right auricle to the left ventricle, by the medium of the lungs, is manifest from the structure of the heart itself. The valves, which are placed at its various apertures, actually will not admit of the blood's motion in any other direction than what we have described. That this fluid passes from the heart into the trunk of the aorta, thence into its branches, and so on to the most minute ramifications, is evinced; 1. By the effect of ligatures on these vessels; the artery becomes turgid

between the heart and the ligature, and empty between the ligature and its distribution. 2. By opening an artery when tied, above and below the ligature; the blood in this case flows from that opening only, which is nearest to the heart. 3. By ocular testimony; the passage of the blood can be seen with the aid of glasses in frogs, fishes, &c. The passage of the blood through the veins, in a contrary course to that, in which it flows along the arteries, i. e. from the minute ramifications towards the trunks, and thence to the heart, is proved. 1. By the structure and disposi tion of the valves, which afford an invincible impediment to all retrograde motion. 2. By ligatures on these vessels, which make the vein turgid between the extremities of the body and the ligature, and empty in the rest of its course. 3. By opening a vein, when tied, above and below the ligature. 4. By microscopical observation in animals.

The passage of the blood from the arteries into the veins seems to flow as a corollary, from what we have stated concerning the proofs of its course in these two systems of vessels. We have shewn that the ulti mate arteries are continuous with the ori. gins of the veins; that the blood moves from the heart to the extremities in the former vessels, and that it passes from the extremities to the heart in the latter. The intermediate passage is a direct conse. quence of these facts. But it may be demonstrated by direct proofs independently of this argument. If we tie the artery of a part, its correspondent vein receives no blood; if we take off the ligature the vein is again filled. The quantity of blood expelled from the aortic ventricle is so considerable, that the supply can only be kept up by its return to the heart. We calculate that two ounces are sent into the aorta at each pulsation; if we suppose 80 pulsations in a minute, 9,600 ounces will be thrown out in an hour; and 14,400 pounds in a day. The same blood, therefore, which the aorta received from the heart, must be returned to this viscus; and the only passage, by which it can return, is through the veins. Lastly, the passage of the blood from the arteries into the veins, may be proved by the direct testimony of the senses in living animals. The use of the microscope affords this proof in the transparent parts of cold blooded animals, as the mesentery and web of the foot in frogs, the tail of fishes, &c.