CIRCULATION, RESPIRATION AND EXCRETION 127 



arteries. To prevent any possible flowing back of the blood, 

 the veins are provided at intervals with valves which permit the 

 blood to pass toward the heart 

 but not in the opposite direc- 

 tion. The smaller veins unite 

 to form larger ones, and finally 

 empty their contents through 

 two branches into the right 

 auricle of the heart. From 

 the capillaries of the intestines 

 the blood carrying the re- 

 sorbed nutrients passes through 

 the portal vein, s, to the liver, p, 

 is there distributed through 

 another system of capillaries 

 and then rejoins the blood 

 from the extremities through 

 the hepatic vein, u. Into the 

 branch, k, coming from the 

 head and anterior parts of the 

 body, the nutrients which are 

 resorbed by the lacteals enter 

 by way of the thoracic duct. 

 183. Course of the blood. - 

 The blood returning through 

 the veins from the extremities 

 of the body to the heart enters 

 first the right auricle (a, Fig. 

 1 8), through two large veins, 

 k and /, coming from the an- 

 terior and posterior parts of 

 the body. The auricle then 

 contracts and the blood, being 

 prevented from returning into 

 the veins by the valves at their 



mouths, is forced through the FlG - l8 - ~ Scheme of circulation of 

 , . ' ,, .... 11 , blood. (Armsby, Manual of Cattle 



valve in the partition wall into F ee ding.) 



the right ventricle, b. This, 



in turn, contracting, the blood, prevented as before by a 



valve from turning back in its course, is forced out of the 



