THE HEART. 499 



Ck The second extends from all parts of the body to the lungs, and carries 

 dark blood. 



" The third passes from the majority of the organs towards the canal 

 carrying dark blood, in which it terminates ; it conveys the white blood or 

 lymph. 



" The red-blood and dark-blood canals bear the greatest analogy to each 

 other. Both are simple in their middle portion, which alternately dilates 

 and contracts to impress upon the blood the movement necessary to life. 

 Both present at their extremities innumerable ramifications, which ultimately 

 join each other ; so that the fluid they carry passes from one to the other 

 in a constant and circular direction. Both are composed, at their origin, 

 of vessels in which the blood moves in confluent columns : these are the 

 veins ; and in their terminal part, of vessels in which the same liquid is 

 spread in divergent columns : these are the arteries. 



" The canal for white blood is composed of a single order of vessels, the 

 lymphatics : converging tubes, whose common trunk opens into the circulatory 

 canal that results from the abouchement of the red and dark blood canals ; 

 the relation it affects with these latter is that of a tangent with its cir- 

 cumference." Sappey. 



These three canals constitute the circulatory apparatus. 



This apparatus therefore comprises : 1, The heart, a central organ, 

 charged to propel the blood ; 2, A system of centrifugal vessels, the arteries, 

 which carry the blood from the heart into the different organs; 3, A 

 system of centripetal vessels, the veins, which bring the nutritive fluid to 

 the heart ; 4, The lymphatics, an accessory centripetal system, destined to 

 convey the lymph into the blood-vascular circle. 



In many anatomical works, the study of this apparatus the heart, 

 arteries, veins, and lymphatics, is designated " angiology." 



FIEST SECTION. 



THE HEART. 



THE history of the heart comprises : 1, A general view of the organ ; 

 2, The study of its external conformation ; 3, Its interior ; 4, Its structure ; 

 5. A description of the pericardium, the serous cavity containing it ; 6, A 

 glance at its physiology. 



1. The Heart as a Whole. (Figs. 230, 234, 258, 259.) 



General sketch. The heart, the central portion of the circulatory 

 apparatus, is a hollow muscle, whose cavity is divided by a thick vertical 

 septum into two perfectly independent pouches. Of these two contractile 

 pouches, one placed on the track of the dark blood, propels it into the lungs ; 

 the other, situated on the course of the red blood, distributes it to all parts 

 of the body. 



Each of these is subdivided into two superposed compartments by a 

 circular constriction, at which is a membranous valve that at certain fixed 

 periods is elevated, and then forms a complete horizontal partition extended 

 between the two compartments. 



The superior compartment receives the convergent or centripetal portion 



