228 THE LYMPHATIC SYSTEM 



continuous with the lymphatic capillaries, but the more recent observa- 

 tions,, represented by those of MacCallum (Johns Hop. Hosp. Bull., 

 1903), seem to show that the capillaries of the lymphatic system, like 

 those of the blood vascular system, form a series of branching channels 

 which are open only toward the veins. According to this conception, 

 therefore, the tissue juices, formerly also considered as lymph, are con- 

 tained within a separate series of channels, the tissue spaces and lym- 

 phatic canaliculi, and they enter the true lymphatics only by processes of 

 osmosis and the secretory activity of the lymphatic endothelia. 



FIG. 236. THE GROWING END OF A DEVELOPING LYMPHATIC VESSEL IN THE SUB- 

 CUTANEOUS TISSUE OF A FETAL PIG. 



The lumen of the vessel has been filled with a dark injection mass. Highly mag- 

 nified. (After MacCallum.) 



The lymphatic capillaries are of rather irregular caliber, generally 

 greater than that of blood capillaries, and possess frequent sinus-like 

 dilatations, which peculiarity is also characteristic of the larger lym- 

 phatic vessels. 



The lymphatic capillaries soon acquire an adventitial sheath of fibro- 

 elastic tissue and pass into the smaller lymphatic vessels. On attaining 

 a size of from 0.2 to 0.8 millimeter their wall is differentiated into the 

 same three coats which are found in the veins. Except for the fact 

 that they contain lymph instead of blood, these vessels closely resemble 

 the small veins, and like some of the latter vessels they possess frequent 

 valves. 



