462 PHYSIOLOGY OP THE DOMESTIC ANIMALS. 



0.0019 parts of dry fibrin. The serum contains 4.T5 per cent, solids 

 (albumen, fats, salts, etc.), and 95.21 per cent, of water. 



The amount of chyle is proportional to the activity of digestion, 

 after a hearty meal the mesenteric vessels and thoracic duct being dis- 

 tended with a milky emulsion, while during fasting they are collapsed 

 and are only seen with difficulty. 



The movement of the chyle is dependent upon a number of causes. 

 In the first place, as mentioned in the description of the mechanisms of 

 absorption of fat, the villi ma}' be regarded as lymphatic hearts, the con- 

 traction of their muscular fibres leading to an onward movement of the 

 contents of the mesenteric lacteals, backward flow being prevented by 

 the valves of these vessels. 



The respiratory movements are also of influence in producing an 

 onward flow of the contents of the thoracic duct. With each act of ex- 

 piration, as may be readily determined by making a fistula of the thoracic 

 duct where it empties into the veins at the root of the neck, there is an 

 acceleration in the flow of chyle, evidently from compression of the duct, 

 while in inspiration this flow is retarded or may be even arrested. In- 

 spiration, therefore, by the production of negative pressure, favors the 

 flow of chyle from the lacteals into the thoracic duct. 



Contraction of the abdominal muscles, and even intestinal peristalsis, 

 will further aid the forward movement of the chyle. So, also, it has 

 been noticed that there is a slight increase in the rapidity of the flow 

 of chyle coincident with each systole of the heart. "While this factor is 

 comparatively insignificant, it acts by the compression of the thoracic 

 duet where it passes under the arch of the aorta. 



Finally, in addition to these influences and to the vis a tergo from 

 continuous absorption by the villi and the propulsion due to their contrac- 

 tion, the most important factors, there is also a vis a f route concerned in 

 the circulation of the chyle. For, as Draper has pointed out, where the 

 thoracic duct empties into the veins, a suction force is exerted on the 

 contents of the lacteals by the passing current of the venous blood, upon 

 the well-known hydraulic principle of Venturi : " If into a tube, through 

 which a current of water is steadily flowing, another tube opens, its more 

 distant end being in communication with a reservoir of water, through- 

 this tube a current will likewise be established, and the reservoir will be 

 emptied of its contents." 



