42 ELEMENTARY PHYSIOLOGY 



indirectly traced, is an organ, the size of which is usually 

 roughly estimated as e([ual to that of the closed fist of the 

 person to whom it belongs, and which has a broad end 

 turned upwards and backwards, and rather to the right 

 side, called its base : and a pointed end which is called 

 its apex, tinned downwards and forwards, and to the 

 left side, so as to lie opposite the interval between the 

 fifth and sixth ribs. 



It is lodged between the lungs, nearer the front than 

 the back wall of the chest, and is enclosed in a sort of 

 double bag— the pericardium (Fig. 13, p.). One-half 

 of the double bag is closely adherent to the heart itself, 

 forming a thin coat upon its outer surface. At the base 

 of the heart, this half of the bag passes on to the great 

 vessels which spring from, or open into, that organ ; and 

 becomes continuous with the otlier half, which l(H)sely 

 envelopes botli tlie heart and the adiierent lialf of the 

 bag. Between the two layers of the pericardium, con- 

 sequently, there is a completely closed, narrow cavity, 

 lined by an epithelium, and containing in its interior a 

 small quantity of clear fluid, the pericardial fluid.' 



The outer layer of the pericardium is firmly connected 

 below with the upper surface of the diaphragm. 



But the heart cannot be said to depend altogether upon 

 the diaphragm for support, inasmuch as the great vessels 

 whicli issue from or enter it — and for the most part pass 

 upwards from its base — help to suspend and keejj it in 

 place. 



Tims the heart is coated, outside, by one layer of the 

 pericardium. Inside, it contains two great cavities or 

 "divisions," as they have been termed above, completely 

 separated by a fixed partition which extends frt)m the 

 base to the apex of the heart ; and consequently, having 



1 This fluid, like that contained in the peritoneum, pleura, and other 

 shut sacs of a similar character to the pericardium, used to be called 

 serum ; whence the membranes forming tlie walls of these sacs are fre- 

 quently termed sr-rnus uteiitOi-anes. The fluid is, however, in reality a 

 form of lymph. (See Lesson III.) 



