78 MANUAL OF OATTLE-i^EEDING. 



Tlie anriele then contracting, sends the blood into tlio 

 left ventricle, g^ wliieli, in its turn, contracts powerfully 

 and expels the blood into one large vessel, the aoHa^ h. 

 The aorta, soon after leaving the heart, divides into two 

 branches, the cmUrior (lorta^ % leading to the fore pait of 

 the body, and the ^posterior dorta, jj supplying the abdom- 

 inal cavity and the posterior part of the body. 



These blood-vessels repeatedly subdivide and carry the 

 blood to all parts of the body, to be brought back again to the 

 right side of the heart and undergo the same process anew. 



The sole cause of the motion of the blood is the power- 

 ful contraction of the muscles of the heart. This alter- 

 nate contraction and relaxation constitutes the beating of 

 tlie he^art, and the sudden impulse thus given to the blood 

 in the arteries causes the beating of the pulse. 



The Arteries, which conduct the blood from the heart 

 to the various organs of the body, are tubes with strong, 

 elastic, and contractile walls, to withstand the force with 

 which the blood is pressed into them by the heart. They 

 originate in the aorta, A (Fig. 4), which receives the 

 blood from the left ventricle, and as they extend farther 

 and farther from the heart throw off branches to the vari- 

 ous organs, become smaller and smaller, and finally end in 

 the eapiUaries, 



The Capillaries are exceedingly fine blood-vessels 

 which penetrate all parts of the body and form the con- 

 necting link between the arteries and veins. Their walls 

 are thin and delicate, and through them the nutritive mat- 

 ters of the blood diffuse out into the tissues to repair their 

 waste, while the worn-out matters, at the same time, dif- 

 fuse into the blood. Thus all parts of the body are kept 

 continually bathed in a solution of nutritive matters. 



In the capillaxies, too, the oxygen which the blood has 



