154 PHYSIOLOGY. 



During the intermission between active digestion the lacteals 

 carry pure lymph, and, from their functions and structure being 

 identical with that of true lymphatics, they deserve to be classed 

 with the latter. 



Origin of the Lymphatics. 



Lymphatic System. 



Miss Florence E. Sabin has shown that the lymphatic system in 

 the embryo pig develops as two blind diverticula from the veins of 

 the cervical and inguinal regions. These grow toward the skin and 

 widen out into four lymph-sacs, from which the final lymphatics pro- 

 ceed. By a special growth of the lymphatics along the dorsal line, 

 the thoracic duct is formed. 



Though many features of this system are yet obscure and open 

 for investigation, it seems very probable that, as stated by Landois, 

 the lymphatics arise as follows: 



1. Connective- tissue Spaces. These are very numerous, star- 

 shaped or irregularly branched spaces which communicate with one 

 another by fine tubular processes. They are lined with endothelium 

 and contain lymph and a few "wandering cells/ 7 



2. Within the Villi. 



3. In Perivascular Spaces. The small blood-vessels which sup- 

 ply bone, central nervous tissue, retina, and the liver are themselves 

 surrounded by lymphatic tubes which, in many instances, are larger 

 than the blood-vessels. Between these tubes and the blood-vessels 

 there exists a space called the perivascular space of His. These are 

 believed to be one source of lymphatics, for, when they exist, the 

 passage of lymph-corpuscles into the lymphatic vessels is greatly 

 facilitated. 



4. In the Form of Interstitial Slits Within Organs. Within the 

 testicle and certain other organs there exist long, slitlike spaces 

 between the various cells and network of tubules. They are all, 

 however, lined with endothelium. Into these spaces there is poured 

 lymph from the blood-capillaries for the maintenance of the gland- 

 ular cells, and at the same time it furnishes material for secretion. 

 From these little slits lymphatics take their origin, but receive inde- 

 pendent walls after their exit from the gland-substance. 



5. By Means of Free Stomata. These occur, for the most part, 

 upon the walls of the larger serous cavities. Lymph is pumped here 

 by the alternate dilatation and contraction of the serous surface, due 



