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food, together, with the refuse of the intestinal secretions. These pass through the ileo-cæcal orifice into the large intestine, and there become reduced to the condition of feces.

The absorption of the digested fluids is accomplished both by the blood vessels and the lacteals. It was formerly supposed that the lacteals were the only agents in this process; but it has now been long known that this opinion was erroneous, and that the blood vessels take at least an equal part in absorption, and are in some respects the most active and important agents of the two. Abundant experiments have demonstrated not only that soluble substances introduced into the intestine may be soon afterward detected in the blood of the portal vein, but that absorption takes place more rapidly and abundantly by the blood vessels than it does by the lacteals. The most decisive of these experiments were those performed by Panizza on the abdominal circulation.' This observer opened the abdomen of a horse and drew out a fold of the small intestine, eight or nine inches in length (Fig. 40, a, a),

Fig. 40.

a

с

PANIZZA'S EXPERIMENT.-aa. Intestine. b. Point of ligature of mesenteric vein. c Opening in intestine for introduction of poison. d. Opening in mesenteric vein behind the ligature.

which he included between two ligatures. A ligature was then placed (at b) upon the mesenteric vein receiving the blood from this portion of intestine; and, in order that the circulation might not be interrupted, an opening was made (at d) in the vein behind

In Matteucci's Lectures on the Physical Phenomena of Living Beings, Pereira's edition, p. 83.

the ligature, so that the blood brought by the mesenteric artery, after circulating in the intestinal capillaries, passed out at the opening, and was collected in a vessel for examination. Hydrocyanic acid was then introduced into the intestine by an opening at c, and almost immediately afterward its presence was detected in the venous blood flowing from the orifice at d. The animal, however, was not poisoned, since the acid was prevented from gaining an entrance into the general circulation by the ligature at b.

Panizza afterward varied this experiment in the following manner: Instead of tying the mesenteric vein, he simply compressed it. Then, hydrocyanic acid being introduced into the intestine as above, no effect was produced on the animal, so long as compression was maintained upon the vein. But as soon as the blood was allowed to pass again through the vessels, symptoms of general poisoning at once became manifest. Lastly, in a third experiment, the same observer removed all the nerves and lacteal vessels supplying the intestinal fold, leaving the blood vessels alone. untouched. Hydrocyanic acid now being introduced into the intestine, found an entrance at once into the general circulation, and the animal was immediately poisoned. The blood vessels, therefore, are not only capable of absorbing fluids from the intestine, but may even take them up more rapidly and abundantly than the lacteals.

These two sets of vessels, however, do not absorb all the alimentary matters indiscriminately. It is one of the most important of the facts which have been established by modern researches on digestion that the different substances, produced by the operation of the digestive fluids on the food, pass into the circulation by different routes. The fatty matters are taken up by the lacteals under the form of chyle, while the saccharine and albuminous matters pass by absorption into the portal vein. Accordingly, after the digestion of a meal containing starchy and animal matters mixed, albuminose and sugar are both found in the blood of the portal vein, while they cannot be detected, in any large quantity, in the contents of the lacteals. These substances, however, do not mingle at once with the general mass of the circulation, but owing to the anatomical distribution of the portal vein, pass first through the capillary circulation of the liver. Soon after being introduced into the blood, and coming in contact with its organic ingredients, they become altered and converted, by catalytic transformation, into other substances. The albuminose passes into the condition of ordinary albumen, and probably also partly into that of fibrin; while the sugar rapidly

becomes decomposed, and loses its characteristic properties; so that, on arriving at the entrance of the general circulation, both these newly absorbed ingredients have become already assimilated to those which previously existed in the blood.

Fig. 41.

The chyle in the intestine consists, as we have already mentioned, of oily matters which have not been chemically altered, but simply reduced to a state of emulsion. In chyle drawn from the lacteals or the thoracic duct (Fig. 41), it still presents itself in the same condition and retains all the chemical properties of oil. Examined by the microscope, it is seen to exist under the form of fine granules and molecules, which present the ordinary appearances of oil in a state of minute subdivision. The chyle, therefore, does not represent the entire product of the digestive process, but contains only the fatty substances, suspended by emulsion in a serous fluid.

During the time that intestinal absorption is going on, after a meal containing fatty

CHYLE FROM COMMENCEMENT OF THORACIC

DUCT, from the Dog.-The molecules vary in size from 1-10,000th of an inch downward.

ingredients, the lacteals may be seen as white, opaque vessels, distended with milky chyle, passing through the mesentery, and con. verging from its intestinal border toward the receptaculum chyli, near the spinal column. During their course, they pass through several successive rows of mesenteric glands, which also become turgid with chyle, while the process of digestion is going on. The lacteals then conduct the chyle to the receptaculum chyli, whence it passes upward through the thoracic duct, and is finally discharged, at the termination of this canal, into the left subclavian vein. (Fig. 42.) It is then mingled with the returning current of venous blood, and passes into the right cavities of the heart.

The lacteals, however, are not a special system of vessels by themselves, but are simply a part of the great system of "absorbent” or "lymphatic" vessels, which are to be found everywhere in the integuments of the head, the parietes of the trunk, the upper and lower extremities, and in the muscular tissues and mucous membranes

throughout the body. The walls of these vessels are thinner and more transparent than those of the arteries and veins, and they

Fig. 42.

LACTEALS, THORACIC DUCT, &c.-a. Intestine. b. Vena cava inferior. c, c. Right and left subclavian veins. d. Point of opening of thoracic duct into left subclavian.

are consequently less easily detected by ordinary dissection. They originate in the tissues of the above-mentioned parts by an irregular plexus. They pass from the extremities toward the trunk, converging and uniting with each other like the veins, their principal branches taking usually the same direction with the nerves and bloodvessels, and passing, at various points in their course, through certain glandular bodies, the "lymphatic" or "absorbent" glands. The lymphatic glands, among which are included the mesenteric glands, consist of an external layer of fibrous tissue and a contained pulp or parenchyma. The investing layer of fibrous tissue sends off thin septa or laminæ from its internal surface, which penetrate the substance of the gland in every direction and unite with each other at various points.

[graphic]

In this way they form an interlacing laminated framework, which divides the substance of the gland into numerous rounded spaces or alveoli. These alveoli are not completely isolated, but communicate with each other by narrow openings, where the intervening septa are incomplete. These cavities are filled with a soft, reddish pulp, which is penetrated, according to Kölliker, like the solitary and agminated glands of the intestine, by a fine network of capillary blood vessels. The solitary and agminated glands of the intestine are, therefore, closely analogous in their structure to the lymphatics. The former are to be regarded as simple, the latter as compound vascular glands.

The arrangement of the lymphatic vessels in the interior of the

glands is not precisely understood. Each lymphatic vessel, as it enters the gland, breaks up into a number of minute ramifications, the vasa afferentia; and other similar twigs, forming the vasa efferentia, pass off in the opposite direction, from the further side of the gland; but the exact mode of communication between the two has not been definitely ascertained. All the fluids, however, arriving by the vasa afferentia, must pass in some way through the tissue of the gland, before they are carried away again by the vasa efferentia. From the lower extremities the lymphatic vessels enter the abdomen at the groin and converge toward the receptaculum chyli, into which their fluid is discharged, and afterward conveyed, by the thoracic duct, to the left subclavian vein.

The fluid which these vessels contain is called the lymph. It is a colorless or slightly yellowish transparent fluid, which is absorbed by the lymphatic vessels from the tissues in which they originate. But little is known regarding its composition, except that it con. tains, beside water and saline matters, a small quantity of fibrin and albumen. Its ingredients are evidently derived from the metamorphosis of the tissues, and are returned to the centre of the circulation in order to be eliminated by excretion, or in order to undergo some new transforming or renovating process. We are ignorant, however, with regard to the precise nature of their character and destination.

The lacteals are simply that portion of the absorbents which originate in the mucous membrane of the small intestine. During the intervals of digestion, these vessels contain a colorless and transparent lymph, entirely similar to that which is found in other parts of the absorbent system. After a meal containing only starchy or albuminoid substances, there is no apparent change in the character of their contents. But after a meal containing fatty matters, these substances are taken up by the absorbents of the intestine, which then become filled with the white chylous emulsion, and assume the appearance of lacteals. (Fig. 43.) It is for this reason that lacteal vessels do not show themselves upon the stomach nor upon the first few inches of the duodenum; because oleaginous matters, as we have seen, are not digested in the stomach, but only after they have entered the intestine and passed the orifice of the pancreatic duct.

The presence of chyle in the lacteals is, therefore, not a constant, but only a periodical phenomenon. The fatty substances constituting the chyle begin to be absorbed during the process of

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