At the entrance of the lateral branches int the thoracic duct, or of one lymphatic into an they, a valve will be found, of a somewhat dill* rent form to those already described. It is corn posed oftwo semilunar flaps, seldom of equal siz arranged somewhat like the ilio.ccecal valve One flap is occasionally so slightly developed that there appears but one large semilunar fold at the entrance of the vessel. At the union of some of these vessels with others, especially of those which lie nearly parallel with each other, no valve will be found, but simply a defined curved line, marking the orifice of communica tion. The valves in the lymphatic system are very closely set together. The distance be tween them varies much. In vessels of a line in diameter they are frequently not more than a line apart, while in others of half that magni tude there may be an interval of an inch between them. It has been observed that they are more frequent and closer together in the larger lym phatics than in the smaller ; this is not always the case ; for instance, the lymphatics of the upper extremity, which are much smaller than those of the lower, have less intervals between their valves ; and neck, where the vessels are still smaller, the valves are less distant apart. It appears to me that the valves are much more approximated to each other in the neighbour hood of the glands, and this observation applies to the vasa afferentia as well as to the vasa effe rentia, but especially to the latter; from which circumstance the notion may have arisen, that the valves are more frequent the larger the vessel. The valves recur less frequently in the thoracic duct than in any other part of the system. It is not uncommon to find in this vessel an interval of two or three inches in extent without a valvular fold.
Mode of origin of the plan I have hitherto adopted in describing the lymphatic vessels has been to present to the reader, first, that which is most readily understood, because easily recognizable by our senses, and about which there could be little difference of opinion; but I have now to direct attention to a part of our subject which has hitherto baffled the efforts of all in viz. the mode of origin of the lymphalic vessels, concerning which the sight, aided by the most powerful glasses, has failed to supply us with satisfactory and demonstrable information. The numerous opinions and conjectures on this sub ject, only present us with so many instances, of the vain struggles of the human mind, to ad vance in a strict science of observation, beyond the limits assigned to our senses, and of the unwillingness, even in the philosopher and man of science, to acknowledge the weakness and limited range of his faculties.
When we consider the transparency of the coats of the lymphatic vessels, as well as of their contents, the small size of their secondary branches, and the numerous valves they every where present, we cannot feel surprised that their precise origin should be involved in much obscurity. From the opaque nature of the chyle, it might be imagined that while the vessels were distended with this fluid, the anatomist would be enabled to trace them to their commencing branches, but unfortunately, it is almost impossible to prevent the onward motion of the chyle in the vessels first re ceiving it, while the valves offer a complete harrier to any retrograde movement. Added to
which, the opacity of the coats of the intestine renders it extremely difficult to follow these vessels from the peritoneal surface of the outer, to the villous surface of the inner tunic, even when distended with chyle. Transparency of the coats of the intestine may be obtained by drying, but the chyle becomes transparent at the same time. Various modes of investigation have been adopted by anatomists to overcome these difficulties, the principal of which it may be necessary here to mention. Injections of quicksilver of coloured fluids have been thrown into the arteries, by which means the injection has occasionally made its appearance in the lymphatic vessels : this result occurs, accord ing to Panizza, in a particular organ in an animal of one species, while in another the experiment will succeed, not in the same, but in some other organ. Thus he succeeded in filling the lymphatics from the arteries, in the intestines of the dog and pig ; in the liver of man, the horse, and dog ; in the testicle of the dog and bull; in the penis and spleen of the horse. Ile was unsuccessful in the intestines of man, the horse, birds, the salamander, and the tortoise ; in the liver of reptiles ; in the spleen of man, the dog, and the pig ; in the kidneys of mammalia and birds; in the penis of man and the dog.
Breschet is of opinion that injections by the veins pass more readily into the lymphatic sys tem. I have now in my possession two prepa rations where the lymphatics have been acci dentally injected from the veins ; one in the mesentery of the turtle, the other in the kidney of man ; and I have undoubtedly observed this occurrence much more frequently after in jections by the veins than by the arteries. Every anatomist who has had much experience in injecting the lymphatic vessels has been in commoded by the injection passing unex pectedly into some large vein, and on looking for the communication, he has found the veins from a lymphatic gland conveying the injection into one of the nearest large venous trunks. This has occurred to me frequently in the human subject, where the common iliac veins and the cava inferior have received the quick silver from the veins of the neighbouring lymphatic glands. I have also seen the same occurrence lately in the horse, and have the specimen shewing the fact now in my museum. It was these veins conveying the injection from the lymphatics into the venous trunks, which Lippi mistook for the vasa efferentia of the glands, and which induced him to publish his work describing many terminations to the lymphatic system in mammalia, hitherto un known to anatomists. This error was the more excusable, inasmuch as his opinion appeared to be confirmed on the investigation being pursued by himself and others in the remain ing classes of vertebrate animals, where various communications do actually take place between the lymphatic trunks and the veins.