CIKCULATION 85 



to contract. 



The rate of respiration is intimately connected with 

 the rate of circulation, through the action of the vagus 

 nerve, which is connected with both. If the respiration 

 becomes slower and deeper, the heart beat becomes cor- 

 respondingly quicker, so that the blood is kept moving- 

 fast enough to get sufficient oxygen for the body. 



If the pressure in the large arteries is lessened by the 

 dilation of the smaller ones, the heart again works 

 more quickly, blood is pushed faster into the large arte- 

 ries, and the pressure is kept great enough to supply 

 the tissues with blood. 



If the heart beats too fast, the extra pressure stimu- 

 lates the depressor nerve and the beat is immediately 

 decreased. 



Summary. — The blood is the great carrier of the body. 

 It transports oxygen and digested food products to tis- 

 sues deficient in them, and it carries the waste products 

 of oxidation from the tissues to the excretory organs. 

 It is a salt solution containing in suspension proteids 

 and living corpuscles. The red corpuscles are important 

 because they act as a storehouse for oxygen, and the 

 white corpuscles are important because they help the 

 body to resist disease and because they assist the clotting 

 of the blood. 



The passage of the blood through the body is deter- 

 mined by five mechanical features. The heart through 

 its rhythmical beat furnishes the motive power. The 

 valves force the blood to go in one direction only. The 

 tubes to which the blood is confined are so arranged that 



