THE HEART. 583 



Both are simple in their middle portion, which alternately dilates and contracts 

 to impress upon the blood the movement necessary to life. Botli present at their 

 extremities innumerable ramifications, which ultimately join each other ; so that 

 the fluid they carry ])asses from one to the otlier in a constant and circular 

 direction. Both are composed, at their origin, of vessels in which the blood 

 moves in confluent colunms — these are the veins ,• and in their terminal portion, 

 of vessels in which the same fluid is spread in divergent columns — these are the 

 arteries (Fig. 349). 



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

 hjmpluitics — converging tubes, the common trunk of which opens into the circu- 

 latory canal resulting from the junction of the red and dark blood vessels ; the 

 relation it affects with these latter, is that of a tangent with its circumference." 

 (Sappey.) 



These three canals constitute the circulatory apparatus. 



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

 propels 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, for conveyance of lymph into the 

 blood-vascular circle. 



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

 veins, and lymphatics — is designated Angiology. 



FIRST SECTION. 

 THE HEART. 



The history of the heart comprises : 1. A general view of the organ. 2. The 

 study of its external conformation. ?>. 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. 255, 349, 350). 



General sketch. — The heart — the central portion of the circulatory apparatus — 

 is a hollow muscle, the cavity of which is divided by a thick vertical septum, 

 into two perfectly independent chambers. Of these two contractile cavities, one — 

 placed on the track of the dark blood— propels it into the lungs ; the other — 

 situated on the coui-se of the red blood — distributes it to all parts of the body. 



Each of these is subdivided into two superposed compartments by a ch'cular 

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

 elevated, and then forms a comple horizontal partition extended between the 

 two compartments. 



The superior compartment receives the convergent or centripetal portion of 

 the blood-canal — that is the veins : it is named the auricle. The inferior gives 

 origin to the divergent or centrifugal part of the same canal, and is designated 

 the ventricle. 



