128 NUTRITION OF FARM ANIMALS 



ventricle into the pulmonary artery, c, which divides into two 

 branches leading to the capillaries of the right and left lungs, 

 d, d. The entrance to this blood vessel, like that of the others, 

 is provided with a valve which prevents the return of the blood. 

 The blood, after passing through the lung capillaries, returns 

 to the left auricle, /, through the pulmonary veins, represented 

 by e. The auricle then contracting, sends the blood into the 

 left ventricle, g, which, in its turn, contracts powerfully and 

 expels the blood into one large vessel, the aorta, h. The aorta, 

 soon after leaving the heart, divides into two branches, i and j, 

 and these repeatedly subdivide, forming the arteries which carry 

 the blood to the arterioles and capillaries, whence it returns 

 again through the veins to the right side of the heart. 



The passage of the blood from the left side of the heart through 

 the body capillaries and back to the right side is called the 

 greater or systemic circulation; that from the right side 

 of the heart through the lung capillaries, the pulmonary cir- 

 culation. 



The appearance of the blood in the veins and arteries is 

 strikingly different. In the veins it has a dark, cherry-red 

 color, but after it has passed through the lungs and is sent out 

 by the heart to the arteries it has a bright scarlet color. The 

 former is called venous, the latter, arterial blood. An exception 

 to this rule, that the arteries carry bright red blood and 

 the veins dark, is found in the pulmonary circulation, where, 

 of course, the vessels leading from the heart to the lungs carry 

 venous blood, and those leading from the lungs to the heart, 

 arterial. Nevertheless, the general nomenclature is adhered 

 to, and the former are called arteries and the latter veins. Ar- 

 teries conduct the blood from the heart, veins toward it. 



184. Mechanics of circulation. While it is not uncommon 

 to speak of the flow of the blood, or of the blood stream, sug- 

 gesting an analogy to a brook or river, the circulation is not in 

 reality a flow of this sort but resembles rather the movement of 

 the water pumped into a hose by a force pump. The heart 

 constitutes the force pump and the arteries correspond to the 

 hose. The powerful muscular contraction of the ventricle 

 drives the blood into the arteries by successive impulses, as the 

 water is driven into the hose by the pump. If the end of the 

 hose were left open the water would issue in a series of spurts 



