ABSORPTION BY LACTEAL AND LYMPHATIC VESSELS. 303 



Physiological Anatomy of the Lacteal and Lymphatic System. One of the most diffi- 

 cult problems in anatomy is to determine the situation and mode of origin of the lym- 

 phatics in different parts of the body. The tenuity of the walls of these vessels, even in 

 their course, and the presence of innumerable valves, render it impossible to study them 

 by the ordinary methods of injection. Since it has been ascertained, however, that they 

 originate in many parts by a rich, anastomosing plexus, their anatomy has been well 

 made out in certain situations by simply puncturing with a fine-pointed canula the parts 

 in which the plexus is supposed to exist, and allowing a fluid, generally mercury, to 

 gently diffuse itself in the vessels of origin. Following the course of the vessels, the 

 fluid passes into the larger trunks and thence to the lymphatic glands. The regularity 

 of the plexus through which the fluid is first diffused and the passage of the injection 

 through the larger vessels to the glands are positive proof that the lymphatics have been 

 penetrated and that the appearances observed are not the result of mere infiltration. 



By the method of investigation above indicated, we may recognize the superficial 

 vessels of the skin, deeper vessels situated just beneath the skin, and vessels in the serous 

 membranes, glandular organs, lungs, tendons, etc., in addition to the larger trunks, such 

 as the thoracic duct. The lacteal system presents essentially the same characters as the 

 general lymphatics, and the vessels are filled with colorless lymph during the intervals 

 of digestion. In many situations, the lymphatics present in their course little, solid 

 structures called lymphatic glands. 



The mode of origin of the finest vessels, in the lymphatic radicles, is exceedingly ob- 

 scure, notwithstanding the numerous investigations which have been made within the 

 last few years, particularly by German anatomists. We shall first describe, however, 

 the mode of origin of what may be called the true vessels, in those parts in which 

 the results of anatomical study seem positive and definite, before we discuss the va- 

 rious theories which have been proposed to account for certain of the phenomena of 

 absorption. 



Lymphatics have not been actually injected and demonstrated in all the tissues of the 

 body ; but, in some parts in which it has been thus far impossible to inject them, we are 

 not justified in assuming positively that they do not exist. For example, in the intestinal 

 villi, according to Sappey, these vessels have never been seen, although their existence is 

 almost certain. The most generally received view with regard to the ordinary mode of 

 origin of the lymphatic vessels is that they commence by a capillary plexus, which does 

 not communicate with either the small arteries, veins, or the capillary blood-vessels, and 

 is generally situated external to the blood-vessels. It does not appear that the vessels 

 composing this plexus vary much in size. They are very elastic, and, after distention 

 by injection, they return to a very small diameter when the fluid is allowed to escape. 

 It is probable, therefore, that the capacity of the vessels is much exaggerated by the 

 means which are taken to render them apparent. In the elaborate observations by Dr. 

 Belaieff, of St. Petersburg, into the origin of the lymphatics of the penis, the walls of the 

 vessels were rendered apparent by the action of nitrate of silver in solution in pure water, 

 and it is probable that they were very little distended. The smallest of these vessels had 

 a diameter of about ^$ of an inch. This may be taken as their average diameter in the 

 primitive plexus. This plexus, when the vessels are abundant, as they are in certain 

 parts of the cutaneous surface, resembles an ordinary plexus of capillary blood-vessels, 

 except that the walls of the vessels are thinner and their diameter is greater. 



The smallest lymphatic vessels are by far the most numerous. They are arranged in 

 the form of a fine plexus, very superficially situated in the skin. A second plexus exists 

 just beneath the skin, composed of vessels of much greater diameter. The skin is thus 

 enclosed, as it were, between two plexuses of capillary lymphatics. A plexus analogous 

 to the most superficial plexus of the skin is found just beneath the surface of the mucous 

 membranes. These may, indeed, be classed with the superficial lymphatics. The deep 

 lymphatics are much larger and less numerous, and their origin is less easily made out. 



