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

 by means of small vessels, called the vasa vasonim. 



412. The Portal Vein (Plate XXXII., 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 cliarged with 

 material newly absorbed, through the walls of the stomach and 

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



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

 the large venae cavs, the dark venous blood reaches the heart ; entering 

 by the right auricle, and, passing through the openmg guarded by the 

 tri-cuspid 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 bi-cuspid 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 vems 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. 



414. 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 go ,, 



