24 RAPID GROWTH OF BIOPLASM. 



verse them freely and return to the same point. This 

 is circulation. In the higher animals and man these 

 tubes and certain organs connected with them, con- 

 cerned in the propulsion of the fluid, are comprised 

 under the head of circulating organs, and the fluid which 

 continues to circulate in the vessels as long as life lasts 

 is called the nutrient circulating fluid, or the llood. 



The Heart. The most important of these organs is 

 the heart, which is a cavity, the walls of which are 

 entirely composed of muscle, alternately becoming, 

 contracted and relaxed sixty or seventy times in 

 a minute, as long as life lasts. When the muscular 

 tissue contracts, the cavity is much reduced in size, 

 and part of its contents are forced out into a vessel 

 continuous with it. The cavity enlarges again during 

 the relaxed state of the muscular walls, and then it 

 receives fluid instead of expelling it. This hollow 

 muscular heart is connected with different sets of 

 tubes or vessels. From one set it receives blood 

 which it drives into the other set of tubes. The part 

 of the heart which receives is called the auricle or 

 receiving cavity, and this opens into the ventricle 

 or propelling cavity. When the latter contracts, the 

 blood is not driven back into the auricle, because 

 some valves between the two cavities are instantly 

 closed, and the whole force of contraction is spent in 

 driving the fluid onwards into the large vessel or 

 artery. Now, in man's heart, there are two auricles 

 and two ventricles. This double heart is necessary 

 for driving the blood through the vessels of the body 

 generally, and through those of the lungs. The 

 right auricle receives the dark blood from the veins ; 

 it then passes into the right ventricle, by which it is 

 driven to the lungs, where it loses its dark colour 

 from parting with carbonic acid, and acquires a 

 bright red hue from gaining oxygen. After tra- 

 versing the lungs, the blood is received by the left 

 auricle from which it passes to the left ventricle. By 



