306 ABSORPTION. 



In the mucous system the lymphatics are very abundant. Here are found, as in the 

 skin, two distinct layers which enclose between them the whole thickness of the mucous 

 membrane. The more superficial of these layers is composed of a rich plexus of small 

 vessels, and, beneath the mucous membrane, is a plexus consisting of vessels of larger size 

 and less numerous. The superficial plexus is exceedingly rich in the mixed structure 

 which forms the lips and the glans penis, and around the orifices of the mouth, the nares, 

 the vagina, and the anus. There are certain mucous membranes in which the lymphatics 

 have never been injected. In the serous membranes, the lymphatics have been demon- 

 strated in great abundance. Lymphatics have been demonstrated taking their origin in 

 the voluntary muscles, the diaphragm, the heart, and the non-striated muscular coats of 

 the hollow viscera, although their investigation in these situations is exceedingly difficult. 



Lymphatics are found coming from the lungs in immense numbers. These arise in 

 the walls of the air-cells and surround each pulmonary lobule with a close plexus. The 

 deep vessels follow the course of the bronchial tubes, passing through the bronchial glands 

 and the glands of the bifurcation of the trachea, to empty into the thoracic duct and the 

 great lymphatic duct of the right side. 



In the glandular system, including the ductless glands, and in the ovaries, the lym- 

 phatic vessels are, as a rule, more abundant than in any other parts of the body. They 

 are especially numerous in the testicle, the ovary, the liver, and the kidney. 



In the substance of the brain and spinal cord, Robin and His have demonstrated a 

 curious system of vessels which entirely surround the capillary blood-vessels and are 

 connected with the lymphatic trunks or reservoirs described by Fohmann under the pia 

 mater. The capillary blood-vessels thus float in a fluid contained in these cylindrical 

 sheaths, which exceed them in diameter by from y^o to 4^ of an inch. These investing 

 vessels follow the blood-vessels in their ramifications, and contain a clear fluid, with bodies 

 resembling the lymph-corpuscles. When Robin first described these vessels minutely, he 

 did not state definitely their physiological relations ; but he has since published a memoir 

 in which he describes them as true lymphatic vessels, analogous to the lymphatics which 

 partly surround the small blood-vessels in fishes, reptiles, and batrachians. In these ani- 

 mals, the lymphatics in many parts nearly surround the blood-vessels, to the walls of 

 which the edges of their proper coat are adherent ; and that portion of the wall of the 

 blood-vessel which is thus enclosed forms at the same time the wall of the lymphatic. 

 This disposition of the lymphatics in the brain and spinal cord would allow of free inter- 

 change, by endosmosis and exosmosis, of the liquid portions of the blood and the lymph. 



The lymphatic vessels from the superficial and deep portions of the head and face on 

 the right side, and those from the superficial and deep portions of the right arm, the right 

 half of the chest, and the mammary gland, with a few vessels from the lungs, pass into 

 the great lymphatic duct (ductus lymphaticus dexter), which empties into the venous 

 system at the junction of the right subclavian with the internal jugular. This vessel is 

 about an inch in length and from one-twelfth to one-eighth of an inch in diameter. It is 

 provided with a pair of semilunar valves at its opening into the veins, which effectually 

 prevent the ingress of blood. 



The vessels from the inferior extremities, and those from the lower portions of the 

 trunk, the pelvic viscera, and the abdominal organs, generally pass into the thoracic duct. 

 In their course, all of the lymphatics pass through the small, flattened, oval bodies, called 

 the lymphatic glands, which are so abundant in the groin, the axilla, the pelvis, and in some 

 other parts. From two to six vessels, called the vasa afferentia, enter these bodies, having 

 first broken up into a number of smaller vessels just before they pass in. They pass out 

 by a number of small vessels which unite to form one, two, or three trunks, generally of 

 larger size than the vasa afferentia. The vessels which thus emerge from the glands are 

 called vasa efferentia. 



The lymphatics of the small intestine, called lacteals, pass from the intestine between 

 the folds of the mesentery to empty, sometimes by one, and sometimes by four or five 



