154 THE CIRCULATION 



front. The base, which, is upward, is attached so as to hold it 

 securely in its place, while the apex is freely movable. In 

 order that loss of power from friction may be obviated, the 

 heart is enclosed between two layers of serous membrane, 

 which forms a kind of sac. This membrane, called the peri- 

 cardium, is as smooth as satin, and itself secretes a fluid in 

 sufficient quantities to keep it at all times well lubricated. 

 The lining membrane of the heart, called the endocardium, is 

 extremely delicate and smooth. (Eead Note 7.) 



19. The Cavities of the Heart, -r The heart is hollow, and so 

 partitioned as to contain four chambers or cavities ; two at the 

 base, known as the auricles, from a fancied resemblance to the 

 ear of a dog, and two at the apex or point, called ventricles. 

 An auricle and a ventricle on the same side communicate with 

 each other, but there is no opening from side to side. It is 

 customary to regard the heart as a double organ, and to speak 

 of its division into the right and left heart. For while both 

 halves act together in point of time, each half sustains an 

 entirely distinct portion of the labor of the circulation. The 

 right side always carries the dark or venous blood, and the 

 left always circulates the bright or arterial blood. 



20. If we examine the heart, we at once notice that, though 

 its various chambers have about the same capacity, the walls 

 of the ventricles are thicker and stronger than those of the 

 auricles. This is a wise provision, for it is by the powerful 

 action of the ventricles that the blood is forced to the most 

 remote regions of the body. The auricles, on the contrary, 

 need much less power, for they simply discharge their contents 

 into the ventricles below. (Figs. 35 and 36.) (See Note 6.) 



7. A Poet's Summary of the Circulation : 



"The smooth, soft air with pulse-like waves 

 Flows murmuring through its hidden caves, 

 Whose streams of brightening purple rush, 

 Fired with a new and livelier blush ; 

 While all their burden of decay 



19. Formation of the heart ? Eight and left heart ? 



20. Capacity of the chambers of the heart ? What wise provision is mentioned ? The 

 auricles ? 



