ABSORPTION BY LACTEAL AND LYMPHATIC VESSELS. 309 



longitudinal or oblique. The fibres of connective tissue are very abundant and loosely 

 unite the vessels to the surrounding parts. The internal and the middle coat are closely 

 adherent to each other; but the external coat may readily be separated from the others. 

 Blood-vessels have been found in the walls of the lymphatics, but, as yet, the presence 

 of nerves has not been demonstrated. 



The walls of the lymphatic vessels are very closely adherent to the surrounding tis- 

 sues ; so closely, indeed, that even a small portion of a vessel is detached with great dif- 

 ficulty, and the vessels, even those of large size, cannot be followed out and isolated for 

 any considerable distance. 



In all the lymphatic vessels, beginning a short distance from their 

 plexu.s of origin, are found numerous semilunar valves, generally ar- 

 ranged in pairs, with their concavities looking toward the larger trunks. 

 These folds are formed of the inner two coats ; but the fold formed of 

 the lining membrane is by far the wider, so that the free edges of the 

 valves are considerably thinner than that portion which is attached di- 

 rectly to the vessel. In some of the vessels, at the point where one lym- 

 phatic communicates with another, there is a valve formed of two folds, 

 one of which is much wider than the other ; but, in the valves situated in 

 the course of the vessels, the curtains are of about equal size. The valves 

 are very numerous in all of the lymphatics, but they are most abundant 

 in the superficial vessels. The distance between the valves is from one- 

 twelfth to one-eighth of an inch, near the origin of the vessels, and from 

 one-quarter to one-third of an inch, in their course. In the lymphatics 

 situated between the muscles, the valves are less numerous. They are 

 always relatively few in the vessels of the head and neck and in all that 

 have a direction from above downward. Although there are a number 

 of valves in the thoracic duct, they are not so numerous here as in the 

 smaller vessels. 



In their anatomy and general properties, the lymphatics bear a close 

 resemblance to the veins. Although much thinner and more transparent, 

 their coats have nearly the same arrangement. The arrangement of 

 valves is entirely the same ; and, in both systems, the folds prevent the FIG. 90. Valves of 

 reflux of fluids when the vessels are subjected to pressure. A number 

 of forces (which will be considered hereafter) combine to produce the 

 flow of lymph and chyle in the absorbent system. Among these is intermittent pressure 

 from surrounding parts, which could only operate favorably in vessels provided with nu- 

 merous valves. 



We have already referred to the great elasticity of the lymphatics. It is now pretty 

 generally admitted that the larger vessels and those of medium size are endowed also 

 with contractility, although the action of their muscular fibres, like that of all fibres of 

 the involuntary or non-striated variety, is slow and gradual. Todd and Bowman have 

 demonstrated this property by mechanically irritating the thoracic duct in an animal re- 

 cently killed, but they observed that the contraction was very slow. Milne-Edwards, 

 quoting from a manuscript presented by Colin to the Academy of Sciences, in 1858, states 

 that this observer noted alternate filling and emptying of some of the lacteal vessels in 

 the mesentery of the ox ; portions of the vessels becoming alternately enlarged in the 

 form of pouches, and contracted so that they almost disappeared. There can be no 

 doubt that the lymphatic vessels possess a certain degree of contractility, which is fully 

 as marked, perhaps, as in the venous system. 



One of the most important points in connection with the physiological anatomy of 

 the lymphatic vessels, and one, indeed, upon which rest our ideas of the mechanism of 

 absorption by these vessels, is the question of the existence of orifices in their walls, which 

 might allow the passage of solid particles or emulsions. The most recent observations 



