and it must be admitted, that we know nothing definite regarding the extreme radicles of the chyliferous vessels. When they become perceptible to the eye, they are observed, as in the subjoined figure, communicating frequently with each other; and forming a minute net-work, first between the muscular and mucous membranes, and afterwards between the muscular and peritoneal, until they terminate in larger trunks a, a, a, a. When they attain the point, at which the peritoneal coat quits the intestine, they leave it also; and creep for an inch or two in the substance of the mesentery; when they enter a first row of mesenteric glands.

From these they issue, of a greater size and in less number; proceed still farther along the mesentery; and reach a second row, into which they likewise enter. From these, again, they issue, larger and less numerous, anastomosing with others; and proceeding towards the lumbar portion of the spine, where they terminate in a common reservoir,—the reservoir of Pecquet, the receptaculum or cisterna chyli,—which is the commencement of the thoracic duct. This reservoir is situated about the third lumbar vertebra; behind the right pillar of the diaphragm, and the right renal vessels. The chyliferous vessels generally follow the course of the arteries; but sometimes proceed in the spaces between them. They exist in the lower part of the duodenum, through the whole of the jejunum, and in the upper part of the ileum.

A A. A portion of the jejunum.-b, b, b, b. Superficial lacteals.-c, c, c. Mesentery.-d, d, d. First row of mesenteric glands.-e, e, e. Second row.-f,f. Receptaculum chyli.-g. Thoracic duct.-h. Aorta.-i, i. Lymphatics.

They are composed of two coats; the outer of a fibrous and firm character; the inner very thin; and forming, by its duplicatures, what are called the valves. These valves are of a semilunar form, arranged in pairs, and with the convex side turned towards the intestine. Their arrangement appears to be well adapted for permitting the chyle to flow from the intestine to the thoracic duct, and for preventing its retrograde course; but Magendie affirms, that their existence is by no means constant. Some anatomists describe an external coat, which is formed of condensed cellular tissue, and unites the chyliferous vessels to the neighbouring parts. The mesenteric glands or ganglions are small, irregularly lenticular, organs; varying in size from the sixth of an inch, to an inch; nearly one hundred in number, and situated between the two

laminæ of the mesentery. In them, the lymphatic vessels of the abdomen terminate, and the chyliferous vessels traverse them, in their course from the small intestine to the thoracic duct. Their substance is of a pale rosy colour; and their consistence moderate. By pressure, a transparent and inodorous fluid can be forced from them; which has never been examined chymically. Anatomists differ, with regard to their structure. According to some, they consist of a pellet of chyliferous vessels; folded a thousand times upon each other; subdividing and anastomosing, almost ad infinitum; united by cellular tissue, and receiving a number of blood-vessels. In the opinion of others, again, cells exist in their interior, into which the afferent chyliferous vessels open; and whence the efferent set out. These are filled with a milky fluid, carried thither by the lacteals or exhaled by the blood-vessels.

Notwithstanding the labours of Nuck, Hewson, Abernethy, Mascagni, Cruikshank, Haller, Beclard and other distinguished anatomists, the texture of these, as well as of the lymphatic glands or ganglions in general, is not demonstrated. All that we know is, that the chyliferous and sanguineous vessels become extremely minute in their substance; and that the communication between the afferent and efferent vessels, through them, is very easy; as mercurial injections pass readily from the one to the other.

commencement.

The thoracic duct, g, Fig. 98, is formed by the junction of the chyliferous trunks with the lymphatic trunks from the lower extremity. The receptaculum chyli, already described, forms its After getting from under the diaphragm, the duct proceeds, in company with the aorta, along the right side of the spine, until it reaches the fifth dorsal vertebra; where it crosses over to the left side of the spine, behind the œsophagus. It then ascends behind the left carotid artery; runs up to the interstice between the first and second vertebræ of the chest; where, after receiving the lymphatics, which come from the left arm and left side of the head and neck, it suddenly turns downwards, and terminates at the angle, formed by the meeting of the subclavian and internal jugular veins of the left side.

To observe the chyliferous apparatus to the greatest advantage; it should be examined in an individual recently executed, or killed suddenly two or three hours after having eaten; or in an animal, destroyed for the purpose of experiment, under the same circumstances. The lacteals are then filled with chyle, and may be readily recognised, especially if the thoracic duct has been previously tied.

The chyliferous vessels were unknown to the ancients. The honour of their discovery is due to Gaspard Aselli of Cremona, who, in 1622, at the solicitation of some friends, undertook the dissection of a living dog, which had just eaten, in order to demonstrate the recurrent nerves. On opening the abdomen, he perceived

a multitude of white, very delicate filaments, crossing the mesentery in all directions. At first, he took them to be nerves; but having accidentally cut one, he saw a considerable quantity of a white liquor exude, analogous to cream. ASELLI also noticed the valves, but he fell into an important error regarding the destination of the vessels;—making them collect in the pancreas, and from thence proceed to the liver.

In 1628, the human lacteals were discovered. Gassendi had no sooner heard of the discovery of Aselli, than he spoke of it to his friend Nicholas-Claude-Fabrice de Peiresc, senator of Aix; who seems to have been a most zealous propagator of scientific knowledge. He immediately bought several copies of the work of Aselli, which had only appeared the year previously, and distributed them amongst his friends of the profession. Many experiments were made upon animals, but the great desire of De Peiresc was, that they should be found in the human body. Through his interest, a malefactor, condemned to death, was given up, a short time before his execution, to the anatomists of Aix; who made him eat copiously; and an hour and a half after execution, opened the body, in which, to the great satisfaction of De Peiresc, the vessels of Aselli were perceived, in the clearest manner. Afterwards, in 1634, John Wesling gave the first graphic representation of the chyliferous vessels from the human body; and he subsequently indicated, more clearly than his predecessors, the thoracic duct and the lymphatics. Prior to the discovery of the chyliferous and lymphatic vessels, the veins, which arise in immense numbers from the intestines, and, by their union with other veins, form the vena porta, were esteemed the agents of absorption; and, even at the present day, they are considered, by some physiologists, to participate with the chyliferous vessels in the function:with what propriety we shall inquire hereafter.

The chyle, as it circulates in the chyliferous vessels, has only been submitted to examination in comparatively recent times. The best mode of obtaining it is to feed an animal, and, when digestion is in full progress, to strangle it, or divide the spinal marrow behind the occiput. The thorax must then be opened, through its whole length; and a ligature be passed round the aorta, œsophagus and thoracic duct, as near the neck as possible. If the ribs of the left side be now turned back or broken, the thoracic duct is observed, lying against the œsophagus. By detaching the upper part, and cutting into it, the chyle flows out. In this way, a small quantity only is obtained; but, if the intestinal canal and chyliferous vessels be repeatedly pressed upon, the flow may be sometimes kept up for a quarter of an hour. It is obviously impossible, in this way, to obtain the chyle pure; inasmuch as the lymphatics, from various parts of the body, are constantly pouring their fluid into the thoracic duct.

From the concurrent testimony of various experimenters:—the

chyle is a liquid of a milky-white appearance; limpid and transparent in herbivorous animals, but opaque in the carnivorous; neither viscid nor glutinous to the touch; of a consistence, varying somewhat according to the nature of the food; of a spermatic smell; sweet taste, not dependent on that of the food; neither acid nor alkaline; and of a specific gravity, greater than that of distilled water, but less than that of the blood.

Magendie, and Tiedemann and Gmelin, however, state it to possess a saline taste; to be clammy on the tongue; and sensibly alkaline.

The chymical character of the chyle has been examined by Emmert, Vauquelin, Marcet, and Prout; and is found to resemble the blood greatly. In a few minutes after its removal from the thoracic duct, it becomes solid; and, after a time, separates, like the blood, into two parts, a coagulum and a liquid. The coagulum is an opaque white substance; of a slightly pink hue; insoluble in water; but readily soluble in the alkalies and alkaline carbonates. Vauquelin regards it as fibrine in an imperfect state, or as intermediate between that principle and albumen: but Brande thinks it more closely allied to the caseous matter of milk than to fibrine. The analyses of Marcet and Prout agree, for the most part, with that of Vauquelin. Dr. Prout has detailed the successive changes, which the chyle experiences in its passage along the chyliferous apparatus. In each successive stage, its resemblance to the blood was found to be increased.

Another point of analogy with the blood is the fact,—observed by Bauer, and subsequently by Prevost and Dumas,—that the chyle, when examined by the microscope, contains the same globules as the blood; differing from the latter only in their being but half the size, and devoid of the envelope of colouring matter. Although the chyle has essentially the same constituents, whatever may be the food taken, and separates equally into the clot and the serous portion; the character of the aliment may have an effect upon the portion of those constituents and thus exert an influence on its composition. That it scarcely ever contains adventitious substances we shall see hereafter; but it is obvious, that if an animal be fed on diet, contrary to its nature, the due proportion of perfect chyle may not be formed; and that, in the same way, different alimentary articles may be very differently adapted for its formation. Leuret and Lassaigne, indeed, affirm that in their experiments they found the chyle to differ more according to the nature of the food than to the animal species; but that, contrary to their expectation, the quantity of fibrine, existing in the chyle, bore no relation to the more or less azoted character of the food. They assign it, as constituents, fibrine, albumen, fatty matter, soda, chloruret of sodium and phosphate of lime.

The chief object of Marcet's experiments was to compare the chyle from vegetable, with that from animal, food, in the same animal. VOL. II.

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