CIRCULATION OF DIGESTED FOOD 47 



Proteids constitute about 80 per cent, of the dry matter of the 

 blood (haemoglobin contains proteids). Fats and sugar are 

 also present but generally in sniall amounts in the plasma. 



The Heart. — The blood in the body has to be kept in constant 

 motion, so that the tissues which are depending upon it for their 

 vitality may be continually supplied, and also in order that the 

 impure fluid resulting from these changes, may be rapidly and 

 effectually conveyed to those organs where its purification is 

 carried out. 



Compartments of the Heart. — The heart is the organ which 

 pumps the blood over the body, not only distributing it to the 

 tissues but forcing it on from these back to the heart again to 

 be prepared for redistribution. It may be described as a hollow 

 muscle divided into two compartments, usually described as 

 right and left, but in quadrupeds really anterior and posterior, 

 each compartment being capable of division into an upper or 

 auricle, and a lower or ventricle. Opening into the auricles are 

 large veins which convey the blood back to the heart; from the 

 ventricles other vessels, arteries, take their origin for the con- 

 veyance of blood from the heart ; the two cavities are separated by 

 a valvular arrangement. 



Pure and Impure Blood. — Into the right heart the whole of the 

 impure or venous blood in the body is brought for the purpose 

 of being purified in the lungs; into the left heart the arterial or 

 purified blood is brought back from the lungs for distribution 

 over the body. 



How the Nutrients Enter the Blood. — The absorbed nutritive 

 materials reach the blood when it is returning to the heart. The 

 blood is pumped out through the cavities into the arteries to be 

 sent to all parts of the body, to furnish nourishment for build- 

 ing up the tissues. 



Bespiration. — The blood not only performs the function of 

 supplying nutrient to the tissues, but it also takes up a great 

 deal of the waste matter of the body. This elimination of waste 

 matter by the blood is accomplished to some extent by means of 

 the lungs. The blood is pumped to the lungs and renewed by 

 taking on a fresh supply of oxygen from the inspired air, which 



