394 



BIOLOGY FOR BEGINNERS 



OfAGRAW OF CIRCULATION IN 

 MAMMALS 



VEINS 



ARTERIES 



fpulmone 



specialization. We will recall the likeness between the one-celled 

 protozoan and Crusoe. He had to perform for himself all the 

 functions of life, such as preparing his food, making his clothes 

 and building his home. The higher forms of life are like small 



communities where one 

 man may build the 

 houses or another specia- 

 lize in making clothes. 

 This would correspond 

 to the first steps- in 

 specialization, as shown 

 by sponges, hydra, etc. 

 As the communities 

 grow, many men work 

 together at one trade to 

 supply all, and this 

 would illustrate the 

 grouping of specialized 

 cells into tissues, each 

 performing its function 

 for the whole animal 

 (earth worm). Then in 

 larger communities the 

 wants are more num- 

 erous, more groups of 

 men specialize in dif- 

 fent tato and supp.y 

 others at a distance with 

 their products. This is the stage represented by the higher animals, 

 where a transportation (circulatory) system is required. In man 

 this is accomplished by the blood, which is kept in motion by the 

 heart, and flows through arteries, veins, and capillaries. 



The Blood. The blood is a fluid tissue constituting about T ^ of 

 the weight of the body. It consists of a liquid portion, calle'd the 

 plasma and solid portions, called the corpuscles or blood cells. 

 The plasma constitutes f the bulk of the blood and consists of 



FIG. 127. Diagram showing circulation in 



