Coloured fluids have been thrown into the ca vity of the pleura and peritoneum in living ani mals for the purpose of bringing the lymphatic vessels into view, and of tracing if possible their extreme branches after absorption had taken place. In this way it is said that minute vessels anastomosing with each other, and forming a delicate net-work, may be made apparent on the surface of the serous membrane, and that the trunks of the neighbouring lymphatics may be seen filled with the coloured fluid. In post mortem examinations also, the absorbent vessels have been observed distended with a fluid of a yellow or red colour, where effusions of pus or blood had taken place during life. In all these instances we probably first notice the injection in the larger lymphatic vessels which are easily recognized by their numerous valves, and on tracing these back to their commencing branches we only discover an intricate net-work of mi nute vessels apparently continuous with each other. It is exceedingly difficult to distin guish these from equally small branches of artery and vein, filled probably with the same coloured injection. You look in vain for the channel by which the injection has entered these vessels ; no continuity of lymphatic with the minute twigs of the other sets of vessels can be detected ; no open orifice belonging to either can be distinguished. Many anatom ists have endeavoured to fill the commencing branches of the lymphatic system by forcing' the injection, thrown into them, in a retrograde direction, and in fishes where there are no valves, with the effect of shewing very nume rous lymphatic vessels destitute of orifices, but not so universally distributed as has been imagined.
Fohmann, Breschet, and others have ,simply made a puncture in the tissues, and by forcing quicksilver into the wound, have occasionally succeeded in filling a minute net-work oflympha tics. Cruickshank and Ilewson employed liga tures to the thoracic duct, to the larger lympha tics, or simply round the limb immediately pre vious or subsequent to the death of the animal, for the purpose of distending the radicles of the system. Lastly, the microscope has been had recourse to by most observers. But the pre vailing physiological opinions of the day have had more influence than all our anatomical in vestigations in determining our notions of the mode of origin of the lymphatic vessels. In deed, so much has this been the case, that I shall find it convenient, in treating the subject of the origin of these vessels, to refer to the physio logical views of the periods during which the successive opinions have been broached. The only other observation I shall make on entering upon this difficult and still obscure subject is, that the chyle seen on the coats of the in testine, contained in its proper vessels, so near to the villous tunic, has tempted anatomists to confine their observations perhaps too much to this one-absorbing surface, with the fixed intention of applying the information thus gained, to the whole system ; whereas the fluid contained in the chyliferous vessels differs so much from that of the rest of the system, that it is not very improbable that the former which admit particles of matter should posSCss ori fices, while the latter should receive its con tents by imbibition without perceptible orifices, which, in fact, is the opinion held by two emi nent physiologists, who have paid considerable attention to the subject, Magendie and Cruveil hier. The first opinion with respect to the mode of origin of the lymphatic vessels which I shall consider is that by open orifices.
ftlany investigators at various periods have attributed open orifices to the radicles of the lymphatic vessels; indeed, this has been the prevailing opinion till within the last few years. Asellius, the discoverer of that part of the system which is connected with the intes tines, imagined that his " vasa lactea " com menced by open mouths from the interior of the intestine. His words are, " ad intestina instar hiant spongiosis capitulis." The first dis coverers of the rest of the system, the " vasa lymphatica," did not attribute to them the func tion of absorption, but regarded them as destined to assist the veins in returning the circulating. fluids to the heart. They supposed them, there-' fore, to be continuous with those arteries which admitted a colourless fluid only, while the veins in a similar way received their contents from the arteries conveying the red blood. The lymphatics properly so called were not consi !Pied to possess open orifices at their origin, until they were generally recognized as sharing with the lacteals, the important office of absorb ing fluids, as well as conveying them towards the heart. It was not fairly established until the time of the Hunters, that these vessels formed part of the absorbent system, although Glisson and Hoffmann had expressed their opinion to this effect, a few years after the dis covery of the lymphatic vessels. But to do justice to this part of our subject, it will be necessary to enter more fully into the Hun terian theory of absorption.
The Hunters, Monro, and their followers Cruickshank, Ilewson, and Sheldon, conceived that the lacteals and lymphatics formed one great system of vessels by which alone absorp tion was effected in the living body, either for the purpose of collecting new materials, or for the removal of the old ; consequently, that these vessels were essential agents in the growth and habitual nutrition of the structures ; —that whenever any of the solid or fluid components of the body, whether of a healthy or morbid character, disappeared, their removal was effected by the lymphatic vessels; this in cluded the ulcerative process, which they considered as exclusively carried on by these vessels. Many ingenious experiments were
performed to disprove absorption by the veins. Fluids of various colours impregnated with musk and other odours were thrown into the intestines of living animals, and were after wards detected in the lacteals, but not in the veins, and imbibition was considered impos sible in the living structures. These views were generally received throughout Europe, and have been acquiesced in almost to the present day. Our phraseology, written or oral, whether in reference to Pathology, Physiology, or Anatomy, is evidently still imbued with them. This theory of the func tions of the lymphatic system necessitated a corresponding anatomical disposition in the mode of origin, as well as in the general ar rangement of these vessels. They were re quired to be universal for the purposes of growth and nutrition ; wherever there was the artery to deposit, there must be lymphatic to absorb ; and as imbibition was inadmissible, they were endowed, as a matter of necessity, with open mouths, by which they were said to commence from all serous and from all mucous surfaces, including the interior of all the visceral cavities, the serous linings of the arteries, veins, and even of the lymphatics themselves, the synovial surfaces of the joints ; the surface of the skin, the mucous linings of the alimentary canal, of the aerial, urinary, and other pas sages, of the excretory ducts, and from the in terstitial cellular tissue of the whole organism. It was admitted that the orifices of the lymph atic vessels could not be sliewn ; that they eluded our senses by their transparency and extreme minuteness ; but on the villi of the in testine, the commencing lacteals were supposed to be detected, turgid with chyle, and their mode of origin by patent orifices was described by more than one anatomist. An analogous arrangement at their commencement was ad judged to the lymphatics, without any further investigation directed specially to these vessels. Cruickshank thus describes the appearance of the supposed lacteal orifices on the villi, seen in a female who died suddenly seven or eight hours after a full meal. " In some hun dred villi, I saw a trunk of a lacteal forming or beginning by radiated branches. The ori fices of these radii were very distinct on the surface of the villus, as well as the radii them selves, seen through the external surface pass ing into the trunk of the lacteal; they were full of a white fluid. There was but one of these trunks in each villus." He states also that Dr. Hunter examined them under the microscope, and counted as many as fifteen to twenty orifices to each villus. According to Lieberkuhn, the lacteal commences on the apex of each villus by one or more orifices leading to an ampullula situated near the apex of the villus, from whence one lacteal branch proceeds through the centre to the base of the villus. The ampullula, lie states, is lined by a spongy cellular tissue, which he conceives is subservient to absorption. With respect to the orifices, his words are : " Quod autem unum saltem adsit foraminulum in cujusvis ampul lube spice, certo examine niihi constat : in terdum tamen, licet rarissime, plura ut in pa pillis mammarum, vidisse memini." Sheldon admits Lieberkuhn's description of the orifice of the lacteal vessel, and of the ampullated ap pearance of its commencement from the villus ; but it appears to me, on looking at Shel don's plates, and reading his description of the ampullulw, that he as well as Licberkuhn, whose plates he has copied, have mistaken the mucous follicles of the intestines for the am pullated villi. Speaking of the ampullulw, Shel don says, " I have seen them of different forms, most commonly bulbous, as represented by Lieberkuhn. I have also seen a number of annpullulte filled with chyle, sometimes form ing clusters, as represented in plate I., while in other parts of the small intestines I have found them solitary, and projecting beyond the villi, as may be seen in several of the figures in plate I." Hewson has seen a net-work of lacteals as well as of bloodvessels on the villus, but no ampullul2a; he states that the orifices of the lacteals can only be discerned when the villus is rendered turgid and erect by the fullness of the bloodvessels. In refer ence to these orifices, be says : " It might be here objected that these were only lacerations of the villi, but I am persuaded they were not, from having, on repeatedly examining them, observed the pores or orifices very distinct and empty ; whereas, were they lacerations, I think I should have seen the injection in them, as the villi were so much injected by it." These are the data upon which has been founded the opinion that the lacteals and lymphatics arise every where by open mouths. I have myself examined under the microscope the villi of various animals destroyed at different periods after a meal, for the purpose of detecting the mode of origin of the lacteal vessels. I have looked at them for hours together before and after the bloodvessels had been filled to great minuteness, but have never been enabled to discover orifices on the apices, or on any other part of the villus, and I can adduce the names of a host of modern observers of consi derable celebrity, Itudolphi, Panizza, Haasse, Lamb, Fohmann, Breschet,Miiller,Treviranus, and others, who deny the existence of any such orifices. Magendie and Cruveilliier conceive that the chyle must enter the lacteals by ori.