698 THE LYMPHATIC SYSTEM 



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 centm-y 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 

 Kolliker who, in 1846, saw, with the aid of the microscope, the lymphatic capillaries in the trans- 

 parent 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-capiUaries in the tail of the frog larva, he erroneously sup- 

 posed that the fine processes of the lymphatic capillaries were continuous with similar proc- 

 esses 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 

 CeUular-Pathologie. 



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 fines 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 rep- 

 resent a system ofj irregularly shaped lymph-canaliculi ("Saftkanalchen") which are in open 

 communication on the one hand with the blood-capiUaries, 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." Cohn- 

 heim 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 dis- 

 tinguished 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. AU 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 micro- 

 scope, show that the endothelial fining of these capillaries is complete, with no trace of an open- 

 ing 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 anas- 

 tomosing plexuses, from which may extend cul-de-sacs, which end bfindly. 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 calibre 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 calibre, 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 (figs. 547, 548). Teichmann found lymphatic capillaries varying in diameter from a few 

 thousandths of a millimetre to one millimetre. In the capsule of the spleen of the cow some meas- 

 ured 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. 



1. 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 papilla;. 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. Lymphatic capillaries 

 of the scrotum are sliown in Fig. 547. 



The conjuntiva, 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 



