The lymphatic capillary, like the blood-capillary, is the portion of the lymphatic system which is chiefly concerned in the specific function of this system. In the blood-capillaries, where the blood is separated from the outside tissues by a single layer of flat endothelial cells, there occurs the interchange of fluid substances and of cells, while the heart, arteries and veins serve to transport the blood, modified in the capillaries, to other parts of the body. Similarly in the lymphatic system, it is in the capillaries, both those most peripheral and those in the lymph nodes, where the absorption and interchange of fluid substances and of cells takes place. Consequently it becomes of prime importance to obtain a clear understanding of the structure of the lymphatic capillaries, their relation to the other tissues, and their mode of functioning. At the outset, however, it must be admitted that our knowledge on this subject is far from complete.

 

Historical

Previous to the development of microscopic anatomy, in the middle third of the 19th century, there was no accurate knowledge of such small structures as the lymphatic capillary. In order to explain the absorption of substances by the lymphatics, as well as the passage of substances from the blood-vessels through the tissues, various theories were invented. Prominent among such theories was that of the "vasa serosa", of H. Boerhaave and other 18th century anatomists and physiologists, which was perhaps most elaborately developed by Bichat, 1801-03. According to this theory there are two sets of minute vessels, too small for the passage of cellular elements. The one set leads from the blood-capillaries onto the various surfaces of the body and into the loose spaces in the tissues - the "exhalants". The other set leads from the body surfaces (including the serous cavities) and the loose spaces in the tissues to the lymphatics - the "inhalants" or "absorbents", which take in fluids by a sucking action.

This theory was somewhat shaken by the discovery of Magendie, in the first decade of the 19th century, that absorption may take place by the veins, as well as the lymphatics, and by the criticism of early 19th century anatomists who developed the technic of injection of lymphatics to a high point.

Our present conception of the lymphatic capillaries may be said to have started with Kölliker who, in 1846, saw, with the aid of the microscope, the lymphatic capillaries in the transparent tails of living frog larvae. He found them to be definite structures made up of a thin wall, from which projected fine-pointed processes, and in which were nuclei. Like Schwann who, in 1837, had studied the blood-capillaries in the tail of the frog larva, he erroneously supposed that the fine processes of the lymphatic capillaries were continuous with similar processes of the surrounding connective-tissue cells. Since, according to the conception current at the time, cells were thought to be hollow structures, with a membranous wall and fluid content, it was concluded that the mode of transmission of fluid from blood to lymphatic capillary took place through canaliculi inside these cells. This conception was elaborated by Virchow, in his Cellular-Pathology.

In 1862 von Recklinghausen by means of the silver nitrate staining method discovered that the lymphatic vessels are lined with an endothelium made up of flattened cells whose outlines show as fine dark lines after this treatment. Again, however, as a result of the eagerness to find open passages through the tissues from blood to lymphatic capillary, an erroneous interpretation was made, von Recklinghausen held that the unstained parts outside the lymph vessels represent a system of irregularly shaped lymph-canaliculi ("Saftkanalchen") which are in open communication on the one hand with the blood-capillaries, and on the other with the lymphatics This conclusion has since been disproved by numerous investigators.

In a second series of observations, von Recklinghausen brought evidence in favor of open communications between the lymphatics and the peritoneal cavity. He watched, under the microscope, the passage into lymphatics, through minute openings, of milk, placed on a portion of the central tendon of the diaphragm. These minute openings he termed "stomata". Cohnheim described similar though smaller openings in blood-capillaries, and His described them in other lymphatic capillaries. Arnold termed the openings in the vessels "stigmata," as distinguished from the openings into the peritoneal cavity, or "stomata."

With the advent into microscopical technic of the various dyes for staining cell-nuclei and protoplasm, and the more precise methods for making histological studies, the endothelial wall of the lymphatic capillary has been definitely established, although much remains to be learned concerning the differences between the lymphatics of the various tissues.

Moreover, recent investigators have failed to find open connections between the lumen of the lymphatic vessel and the tissue outside. Kolossow failed to find the "stomata" of von Recklinghausen and the "stigmata" of Cohnheim, His and Arnold. The "stomata" have been carefully studied by a number of other recent investigators. All agree in finding a complete endothelial lining for the lymphatic capillaries lying underneath the peritoneum and pleura, with no openings or "stomata." Careful studies of the lymphatic capillaries in the transparent tails of living frog larvae, which may be clearly seen with the higher magnifications of the microscope, show that the endothelial lining of these capillaries is complete, with no trace of an opening into the spaces in the tissue outside (E. R. Clark).

Form

The shape of the lymphatic capillaries has been found to vary enormously in the different parts of the body, where they have been studied. In general they form richly anastomosing plexuses, from which may extend cul-de-sacs, which end blindly. Such cul-de-sacs are especially noticeable in the dermal papillae, in the filiform papillae of the tongue, and in the intestinal villi. The plexuses are often present in two layers - a superficial and a deep. The vessels of the superficial plexus are of smaller caliber than those of the deep. These two sets of plexuses are particularly well seen in the skin and the gastro-intestinal tract. In relation to the blood-capillaries, the lymphatic capillaries are generally the more deeply placed.

In caliber, unlike the comparatively uniform diameter of blood-capillaries, the lymphatics vary enormously. In the same capillary a very narrow part may be succeeded by a very wide one. Teichmann found lymphatic capillaries varying in diameter from a few thousandths of a millimeter to one millimeter. In the capsule of the spleen of the cow some measured more than 1.5 mm.The capillaries are without valves.

Activity

That the lymphatic endothelium is not exclusively a passive membrane has been shown by Clark in studies on the lymphatics in the transparent tails of living frog larvae. The lymphatics here are seen to send out protoplasmic processes which, somewhat like an amoeba, actively take into the interior of the lymphatic red blood-cells accidentally forced from the blood-capillaries into the tissue-spaces.

The mode of passage of leucocytes into or out of the lymphatics offers no such difficulties as that of the fluids, for they are able, through their power of amoeboid movement, to pass independently through the endothelium - a process first directly observed by Cohnheim.

The Extent and Character of Lymphatic Capillaries

The skin over the entire surface of the body is richly provided with lymphatic capillaries. They form two sets of plexuses in the dermis, a superficial and a deep. The superficial set sends out blind cul-de-sacs into the dermal papillae. The richest skin plexuses are found in the scrotum, the palms of the hand and palmar side of the fingers and in the soles of the feet and plantar side of the toes. In the loose subcutaneous fascia, according to Teichmann, there are present only the larger collecting vessels, with no lymphatic capillaries.

The conjunctiva, both the sclerotic and corneal, is supplied with a rich plexus of capillaries, which are narrower in the corneal than in the sclerotic portion. At the corneal border the capillaries form a fairly regular ring which has been called by Teichmann a circulus lymphaticus.

At the various orifices of the body, the skin plexuses go over into the mucous plexuses, forming anastomoses with them. Throughout the entire alimentary tract, including the nasal cavities, the lymphatic capillaries form extensive plexuses which are in many places divided into a superficial plexus in the mucosa and a deeper plexus in the submucosa. In portions provided with a peritoneal covering, there is a third rich subserous plexus. In the tongue and the small intestine the plexus in the mucosa sends out blind cul-de-sacs; in the tongue into the filiform papilla; in the small intestine into the villi. Where muscle is present along the alimentary tract, the lymphatics pass between the muscle bundles, but form no plexuses around them.

The lining of the tracheal and bronchial passages is supplied with a double plexus of lymphatic capillaries, a mucous and a submucous set, which vary in richness according to the looseness of the tissue. In the smaller bronchi but a single layer of capillaries is present, and, according to Miller, no capillaries are present around the air cells. Plexuses surround the pulmonary arteries and veins. Under the pleura lie rich plexuses which connect with the deeper lymphatics around the veins only in places where the veins reach the surface of the lung.

Concerning the arrangement of the lymphatic capillaries in the glands derived from the alimentary tract much remains to be learned.

The salivary glands have been recently studied anew by Aagaard, who has found lymphatic capillaries accompanying the blood-vessels into the interior of the lobules, and forming here irregular plexuses.

The thyroid gland contains lymphatic plexuses which lie in relation to the colloid-containing alveoli. Direct connection between the lymphatics and the alveoli has lately been described by Matzunaga, but this observation needs verification. The lymphatics are apparently concerned in the absorption of the colloidal secretion, for traces of it have been found in the lymphatics draining the gland.

Concerning the lymphatics of the parathyroids nothing is known.

The course of the lymphatics draining the thymus has been recently described, but the nature of the capillaries in this gland is unknown.

The lymphatic capillaries of the liver are of great importance, for the lymph which flows from this organ forms a very considerable part of the total lymph which is collected into the thoracic duct. And yet very little is definitely known about the name and distribution of the lymphatic capillaries in the interior of the organ. In the capsule there is a rich plexus, lying under the peritoneum, in which very large widenings have been described (called by Teichmann "Lymphbehalter"). In the interior rich plexuses surround the branches of the hepatic artery and portal vein, and plexuses have been described accompanying the branches of the portal vein into the lobules.

The linings of the large bile-ducts and the gall-bladder are provided with a submucous network of lymphatics (Sudler and Clermont). The gall-bladder has also a rich subserous plexus.

Concerning the lymphatic capillaries of the pancreas Bartels notes briefly that they form richly branched plexuses in the interlobular connective tissues, which surround larger or smaller parts of whole lobules, not the single gland elements.

The mucous lining of the genito-urinary tract, wherever- it has been carefully studied has been found provided with plexuses of lymphatics. In the bladder they form a rich plexus of irregular capillaries which lie immediately under the almost intraepithelial blood-capillaries. They connect, through the muscular layer, with a subserous plexus. The lymphatic plexus of the urethra anastomoses with the capillaries of the base of the bladder, and in the male with those of the glans penis. The lymphatic capillaries of the ductus deferens and of the seminal vesicles have not been studied. In the prostate (Camineti) the lymphatics form rich plexuses surrounding the glands, which connect with a very wide meshed subcapsular plexus, surrounding the entire gland.

In the testis there is a rich superficial plexus, lying directly beneath the tunica albuginea. Concerning the deep lymphatics of the testis there has been much dispute. Ludwig and Thomsa found the lymphatic capillaries going over into lacunse, without endothelium. This has been disputed by Tommasi and Gerster, who find, in the septa, capillaries with endothelial wall, which they consider the beginnings of the lymphatics.

In the female, lymphatic plexuses have been found in the mucosa of vagina and hymen, anastomosing with those of the vulva. In the uterus, capillaries in the mucosa are very difficult to demonstrate. Definite lymphatics, however, have been found passing through the muscularis, and under the peritoneum a rich subserous plexus of capillaries is present. In the pregnant uterus these subserous capillaries are much distended (Schick). The Fallopian tubes are provided with lymphatics, but they have not been carefully described.

The ovary has a rich superficial lymphatic plexus. In the interior of the gland, according to His, the capillaries form networks in the connective-tissue framework. In the tunica externa of the follicles there is a rich plexus.

The kidney has two sets of lymphatics, a superficial, capsular set, and a deep set. The capsular set is divided into two layers, one lying directly beneath the peritoneum made up of a wide meshed plexus, and the other in the fibrous capsule of the kidney, with finer capillaries and narrower meshes, which anastomose with the deeper capillaries. The lymphatic capillaries of the kidney parenchyma have recently been described by Kumita. He found rich plexuses in both cortex and medulla, surrounding the straight and convoluted tubules, the loops of Henle and the collecting tubules. He also found a plexus surrounding and accompanying the blood vessels into the interior of the glomeruli.

The lymphatic capillaries of the adrenal have also been described recently by Kumita. His results agree with those of Stilling, who studied the lymphatics of the adrenal of horse, cow and calf. Like the kidney, the adrenal possesses a superficial and a deep set. The superficial set is in two layers, as in the kidney, the outer lying in the looser tissue around the adrenal and the inner lying within and just under the capsule. The latter is made up of a rich lymphatic plexus, which anastomoses with the capillaries of the parenchyma. The parenchymatous lymphatics are present in the form of plexuses which surround the groups of cells.

In spite of numerous investigations, endothelial-lined lymphatics have not been definitely found in the central nervous system, or in the peripheral nerves. The subarachnoid and similar spaces, including the perineural spaces, do not form parts of the lymphatic system.

Rich plexuses of lymphatic capillaries are present in the tendons of muscles (Schweigger- Seidel and Ludwig). In muscles, themselves, the question of the presence of lymphatics has long been disputed, sometimes answered in the affirmative, more often in the negative. A recent study by Aagaard, however, would seem to place beyond doubt the presence of lymphatic capillaries in striated muscles. By long continued injection, he was able to find lymphatics in the intramuscular portions of the tendons, which extended out among the muscle fibers themselves. He also found capillaries in the tongue musculature.

The heart is provided with a subpericardial plexus of lymphatic capillaries. A subendocardial plexus has also been described (Sappey, Rainer). Bock has recently found that there is an extremely rich lymphatic network throughout the substance of the heart. According to his description, the lymphatic capillaries are more numerous than the blood-capillaries. The periosteum of bones is provided with a rich plexus of lymphatic capillaries. They are present in 'several layers, of which the outermost form the richest plexus. Lymphatic capillaries have also been described accompanying the blood-vessels in the Haversian canals in bones (Rauber, Schwalbe, Budge). Nothing is known concerning the lymphatics of the bone marrow. Cartilage lacks both blood and lymphatic capillaries.

The capsular membranes of joints are richly provided with lymphatic capillaries (Tillmanns). They are arranged in two layers - an inner layer made up of a rich plexus of wide capillaries, lying just outside the subendothelial blood-capillaries, and an outer layer, consisting of a rich plexus in the subsynovial tissue. The lymphatic capillaries have no open connection with the joint cavity.

The membranes surrounding the pleural, pericardial and peritoneal cavities are richly supplied with lymphatic capillaries, which form here thick plexuses outside the endothelium. These plexuses are usually described with the underlying organ, as the subserous lymphatic capillaries of the intestine, etc. In the central tendon of the diaphragm the subperitoneal lymphatics are extremely rich. They widen out here to form very large endothelial-lined cavities which, in the spaces between the connective-tissue bundles, lie directly in contact with the peritoneal epithelium. The existence of open connections between these capillaries and the peritoneal and pleural surfaces (the "stomata" of von Recklinghausen) has recently been disproven. The capillaries on the two surfaces of the central tendon communicate freely with one another.

From Morris's treatise on anatomy

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