4° 



CLASSIFICATION 



brought about by squeezing with the hand. An actual heart 

 requires, of course, no squeezing from outside, its walls moving in 

 such a way that the internal cavity is alternately diminished and 

 increased in size. This is due to the fact that its substance is 

 made up of slender muscular fibres arranged in a very complicated 



fashion. Each fibre 

 possesses the power 

 of contraction, i.e. it is 

 able to shorten itself, 

 becoming at the same 

 time thicker. It is 

 clear, therefore, that 



A. 



Base 



ARTERi-^ if a large number of 



such fibres arranged 



^*'-^^s' '^A^vEs transversely and ob- 



Fig. i6. — Diagrammatic Hearts, a, One-chambered; B, two-chambered. i. i i 



liquely to make up 

 the wall of a hollow structure contract at the same time, the result 

 will be that the contained cavity becomes smaller, regaining its 

 original size when the fibres again relax. This is what happens in 

 the case of a heart, enabling it to do pumping work. 



An advance upon the simple one-chambered heart as just 

 described is found in such a case as the common garden snail, 

 where there are two chambers (fig. i6), one a thin-walled auricle, 



receiving blood from the vein, and squeez- 

 ing it on into a thick -walled ventricle, 

 which does the pumping work. 



The heart of a human being, however, 

 has two sets of pumping work to do — ( i ) it 

 forces pure blood all over the body, and 

 (2) it drives impure blood to the lungs for 

 purification. It is therefore not surprising 

 that in such a heart there are four cham- 

 bers, right and left auricles, and right and 

 left ventricles, disposed as shown in the 

 accompanying diagram (fig. 17). The right side of the heart 

 contains only impure blood which it receives from the great veins, 

 which open into the right auricle. This chamber passes the 

 blood on to the right ventricle, which pumps it through a large 

 vessel (pulmonary artery) to the lungs. These purify the blood, 

 and from them it is conducted through pulmonary veins to the left 



Fig. 17. — Diagram of Auricles and 



Ventricles 

 R A and LA, Right and left auricles: 

 E V and L V, right and left ventricles. 



