54 



HUMAN BIOLOGY 



passing through the capillaries of the lungs, the blood goes 

 throngJi tJie pulmonary veins to the left auricle (Fig. 54), 

 then through the bicuspid or mitral valve, to the left ventri- 

 cle, whence it is forced through a semilunar valve into the 

 largest artery of the body, called the great aorta (Fig. 54). 

 Thence it goes to the smaller arteries, and then to the capil- 

 laries of the tissues in general, thus completing the circuit 



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FIG. 55. THE LEFT SIDE OF HEART (plan), showing the left ventricle at the mo- 

 ment when relaxing and receiving the blood from the auricle; and the same at 

 the beginning of contraction to send blood into aorta. Notice action of the valve. 



Structure of Veins and Arteries. Seen under the micro- 

 scope the arteries and veins show that they are made of 

 three kinds of tissues arranged in three coats (Fig. 56) : a 

 tissue resembling epithelial tissue (Chap. I), as a lining 

 to lessen friction ; an outer connective tissue (Chap. I), to 

 give elasticity ; and a middle coat of muscular tissue to 

 enable the vessels to change in size. Let us see why blood 

 vessels must have these three properties? 



Why the Blood Vessels must be Elastic. - -The aorta and its branches 

 are always full of blood. When the left ventricle with its strong, mus- 

 cular walls contracts, the blood in the aorta and small blood tubes can- 

 not move forward fast enough to make room for the new supply so 

 suddenly sent out of the ventricle. Where can this blood go ? If a 



