ANATOMY OF THE LACTEAL AND LYMPHATIC SYSTEM. 431 



the deeper veins in their course. They receive the lymph 

 from the superficial vessels. 



!N"o valvular arrangement is found in the smallest lym- 

 phatics ; but the vessels coming from the primitive plexuses, 

 and the large vessels, contain valves in immense numbers. 

 These valves, being so closely set in the vessels, give to them, 

 when filled with injection, a peculiar and characteristic bead- 

 ed appearance. 



The course of the lymphatics is generally tolerably direct. 

 As they pass toward the great trunks by which they com- 

 municate with the venous system, they present a peculiar 

 anastomosis with the adjacent vessels, called anastomosis by 

 bifurcation ; that is, as a vessel passes along with other ves- 

 sels nearly parallel with it, it bifurcates, and the two branches 

 pass into the nearest vessels on either side. These anasto- 

 moses are quite frequent and generally occur between vessels 

 of equal size. In their course, the vessels pass through the 

 lymphatic glands, which will be described further on. 



A notable peculiarity in the lymphatic vessels is that they 

 vary very little in size, being nearly as large at the extremities 

 as they are near the trunk. In their course, they are always 

 much smaller than the veins and do not progressively enlarge 

 as they pass on to the great lymphatic trunks. The largest- 

 sized vessels as they pass from the skin are from ^ to ^ of 

 an inch in diameter, 1 and the larger vessels, in their course, 

 have a diameter of from y^ to J of an inch. 2 As in the case 

 of the smallest lymphatics in the primitive plexus, the elasti- 

 city of the walls of the vessels renders their calibre greatly 

 dependent upon the pressure of fluid in their interior. Many 

 anatomists have noticed that vessels hardly perceptible while 

 empty are capable of being dilated to the diameter of half 

 a line or more, returning to their original size as soon as the 

 distending fluid is removed. 3 



1 BELAIEFF, op. cit. 



3 KOLLIKER, Manual of Human Miscroscopic Anatomy, London, 1860, p. 503. 



a MILNE-EDWARDS, Lcfom sur la Physiologic, Paris, 1859, tome iv., p. 509, note. 



