THE CIRCULATION' 161 



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- 

 c<ir<Hnm, 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. (Head Note 7.) 



% 



21. The Cavities of the Heart. 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. 



22. 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.) 



The two ventricles,- moreover, are upon different levels. 

 The right ventricle is somewhat in front of and above the 

 left, so that, in viewing the heart from the front, we would 

 look at the right ventricle, and in viewing the heart from be- 

 hind, at the left ; the left ventricle, being longer and narrower 

 than the right, and projecting below the right, forms the apex 



21. Formation of the heart ? Right and left heart ? 



82. Capacity of the chambers of the heart ? The auricles f 



