THE CARDIAC CAVITIES. 



323 



Fig. 93. — Diagrammatic representation of the heart, the great vessels, the pulmonic 

 and systemic capillaries. 



In this diagram, as in Fig. 91, the heart is seen from its dorsal (posterior) aspect ; hence 

 Us right and left portions correspond in position with the right and left of the observer (§ 56). 



Most of the parts are shown by outlines only, but the ventricular and auricular walls 

 are shaded, and the lines representing the aorta and pulmonary artery are made a little 

 heavier than those representing the vena cava and pulmonary vein. 



The course of the blood is indicated by arrows, but the action of the parts will be 

 considered in Part III. The diflFerence between the valves upon the two sides will be 

 explained in connection therewith. 



Uracde^ 



Pig. 92. — Diagram op the Cardiac Cavities, etc. ; Doesal Aspect. 



§ 845. The special objects of this diagram are : — 



1. To represent the more essential parts of the heart in a single figure, to give their 

 full technical names, and to indicate the relations of the cavities to each other and to the 

 great vessels. 



2. To illustrate the physiological fact that the mammalian heart, in the normal adult, 

 really consists of two organs whose cavities have no direct communication whatever. 



3. To show the many points of resemblance between the right and left portions. 



4. To indicate the relations of the two portions of the heart, through their afferent and 

 efferent vessels, with the lungs and the other organs of the body, and thus to justify the 

 apparent paradox that whereas, anatomically, the heart may properly be described as a 

 single and approximately mesal organ, situated between the two lateral masses of the 

 lungs, physiologically, the right and left portions of the heart are separated, on the one hand 

 by the lungs and on the other hy the rest of the body. Anatomically, there are two lungs 



