142 



THE LYMPHATIC SYSTEM 



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 consid- 

 ered as lymph, are contained within a separate series of channels, 

 the tissue spaces and lymphatic canaliculi, and they enter the true 



FIG. 140. SUBCUTANEOUS LYMPHATIC VESSEL OF A PETAL PIG. 



At the right is a small blood vessel. Hematein and eosiu. Highly magnified. (After 



MacCallum.) 



lymphatics only by processes of osmosis and the secretory activity 

 of the lymphatic endothelia. 



The lymphatic capillaries are of rather irregular caliber and 

 possess frequent sinus-like dilatations, which peculiarity is also 

 characteristic of the larger lymphatic 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 mm. their wall is differen- 



