CHAP, vix.] THE CATS ORGANS OF CIBCULATION. 217 



which converge as the corresponding arteries diverge, and empty 

 themselves into the sphniic vein, which returns blood from the 

 pancreas as well as from the spleen, and also a vein from the 

 stomach along its greater curvature. 



The veins thus converging form the portal rein. This portal 

 vein passes to the transverse fissure of the liver above the hepatic 

 artery and duct, enclosed in the gastro-hepatic omentum. Arrived 

 within the fissure, it divides right and left, and ramifies in the right 

 and left i)ortions of the liver, forming the great vessel of each j)ortal 

 canal. The portal vein is without valves. 



§ 24. Another system of venous structures goes by the name of 

 the AZYGOs VEIN, although there are really a pair — one on each 

 side. They are elongated vessels advancing one on each side of the 

 spine, but more or less irregular in form and arrangement. They 

 are formed by the union of the intcrcodal veins (corresponding with 

 the intercostal arteries) and are tolerably symmetrical behind, but 

 anteriorly some of the veins of the left side of the body, though not 

 those of the three or four upper intercostal spaces, are poured into 

 the right azygos vein, which thus becomes enlarged. 



They begin behind in the lumbar veins, and the right azygos rein 

 advances and passes through the diaphragm, with or near the aorta, 

 till near the root of the right lung, where it turns downwards and 

 empties itself into the vena cava superior (Fig. 104, rz), very near 

 the right auricle. 



The /eft azi/gos rein also commences with the lumbar veins, 

 advances and passes through the diaphragm, with or near the aorta, 

 and remaining very slender, ends by opening into the left innominate 

 vein. The azygos veins have valves. 



The veins of the heart itself open into the right auricle between 

 the auriculo-ventricular opening and that of the vena cava 

 inferior. 



Thus the heart may be said to have a small circulation of its own — 

 in addition to the systemic, pulmonary and portal circulations already 

 mentioned. For blood from the coronary arteries * flows from the 

 root of the aorta to the substance of the heart, whence it is directly 

 returned by these coronary veins. 



§ 25. The lipnphatics or ahsorhent vessels (including the alimentary 

 lacteals,) seem to arise by minute vessels in every part of the body, 

 which form a system of tubes (of different sizes,) and reservoirs (or 

 sinuses). 



Some of these latter are much dilated, for the great peritoneal 

 sac, with the other serous sacs — including those investing the 

 nervous centres — are to be regarded as being really large lymphatic 

 sinuses or interspaces, because the lymphatic vessels communicate 

 with the peritoneal cavity by definite apertures called sfomafa. 

 Thus the lymphatic system is not so closed a system as is the 



* See anl'; p. 206, 



