314 IXTEENAL ANAT0M7 OF HORSE. 



in this way completing its circuit. We thus see that there 

 are two systems — one the " pulmonary system," the other the 

 " systemic '' circulation. There is also another system called 

 the "portal" system, which consists of a vein, the portal vein 

 {Pv), whicli runs from the walls of the intestine (/) to the 

 liver {Li). Tliis latter organ is supplied by an artery, the 

 hepatic artery, and a vein, the hepatic vein, besides the portal 

 system. The kidneys also have a renal artery and vein coming 

 from the aorta and going to the vena cava. Lastly, opening into 

 the inferior vena cava near the heart is seen another vessel, 

 the thoracic duct (Td) ; this arises in the walls of the intestines 

 in the so-called lacteal system, and pours the nutritive fluid 

 derived from the food into the blood-system. 



The blood during its passage through the organs oozes 

 through the extremely thin capillary walls. The cells of the 

 organ obtain the nutritive elements from this, and leave behind 

 a watery fluid called lymph. This lymph does not return 

 directly back into the blood, but is drawn away from the organs 

 by a series of tubes which originate as minute blind canals. 

 These lymph-spaces unite into the so-called lymphatic ducts or 

 veins, which, like the blood-veins, are furnished with valves to 

 stop the backward flow of the fluid to the lymph-spaces. 

 Eventually the lymph empties itself into the venous blood- 

 system, and so into the right auricle of the heart. The lacteals 

 and thoracic duct are part of tliis lymphatic system, but 

 specially modified for the conveyance of the chyle to the blood. 

 This lymphatic system is almost as important as the vascular 

 system, for the blood is partly dependent upon it. 



The sources of hlood are five in number, as follows : — 



1. From material absorbed by the lacteals in the primary 

 digestion of food (chyle). 



2. From soluble matters — such as water, sugar, and peptones 

 — absorbed by the blood-vessels, and sent first through the liver. 



3. From matter formed by certain blood-glands, such as the 

 spleen, &c. 



