CIRCULATORY AND LYMPHATIC SYSTEMS 277 



the back-flow of the blood. The walls of the bloodvessels are 

 nourished by means of small vessels, called the vasa vasorum. 



414. The Portal Vein (Plate XXXV., 6 and 7) forms a separate 

 circulatory system. It commences in the sub-lumbar region, being 

 formed by various vessels. It carries blood, which is charged with 

 material newly absorbed, through the walls of the stomach and 

 intestines from the food, and conveys it to the liver. 



415. The Circulation of the Blood. — Through the medium of 

 the large venae cavae the dark venous blood reaches the heart, enter- 

 ing by the right auricle, and, passing through the opening guarded 

 by th Q tricuspid valve, it gains the right ventricle. From thence it 

 is driven, by the contraction of the walls of the ventricle, into the 

 pulmonary artery, and is carried to the right and left lungs, where it 

 gives off carbonic acid gas and becomes charged with oxygen. 

 This action changes its colour from a dark brown to a bright scarlet. 

 The blood is then returned by the pulmonary veins to the left 

 auricle, then through the passage guarded by the bicuspid valves 

 into the left ventricle, whence it is forced into the aorta; thence 

 through the arteries all over the body, carrying to the various parts 

 nourishment, as well as oxygen, to keep up animal heat. When 

 loaded with impurities it is again returned by the veins to the right 

 side of the heart. Thus we have a double circulation, the right 

 side being the venous or pulmonary, while the left is the arterial 

 or systemic. 



416. By the action of the heart — i.e., by the contraction of the 

 ventricles — the blood is forced into the aorta and pulmonary artery ; 

 this causes the arteries to dilate, and this dilatation, running in the 

 form of a wave, pressed forward by the contraction of the arteries, 

 and the force behind it, constitutes the pulse, which corresponds to 

 the beats of the heart. 



The horse's pulse beats from 38 to 43 times per minute. 

 The cow's ,, ,, 50 to 60 



The sheep's ,, ,, 75 to 80 



The pig's ,, „ 70 to 80 



The dog's „ „ 80 to 90 



SJ 5> 



