THE CIRCULATION OE THE BLOOD. 



267 



placed the corresponding auriculo-ventricular opening. The wall 

 of the auricle measures about 3 mm. in thickness. The left ventricle 

 (Fig. 113) is conic in shape from above downward and oval or cir- 

 cular in shape on cross-section. At its upper right angle it presents 

 a circular orifice, the margins of which give attachment to the walls 

 of the aorta, the main arterial trunk of the systemic circulation. The 

 inner surfaces of the ventricle show a similar though better devel- 

 oped system of columnae carneae, musculi papillares, chordae tendineae, 

 etc. The wall of the left ventricle measures about 11.5 mm. in thick- 

 ness in the middle. 



The Endocardium. The cavities of both the right and left sides 

 of the heart are lined by a thin firm connective-tissue membrane, 

 closely adherent to the muscle- 

 tissue, termed the endocar- 

 dium. It also contains elastic 

 fibers and smooth muscle- 

 fibers. Its entire surface is 

 covered over with a layer of 

 polygonal endothelial cells. 

 JThis membrane serves to re- 

 sist undue distention of the 

 heart during contraction and 

 to prevent separation of the 

 muscle-fibers .^The endocar- 

 dium is continuous with the 

 lining membrane of the blood- 

 vessels. 



The Cardio- pulmonary 

 Vessels. Though the two 

 sides of the heart are separ- 

 ated from each other by a 

 vertical septum, they are ana- 

 tomically and physiologically 

 connected by the intermedia- 

 tion of the pulmonary system 

 of vessels: viz., the pulmonary 

 artery, capillaries, and veins 

 (Fig. 114), 



The pulmonary artery arises from the conus arteriosus of the right 

 ventricle. After a short upward course it divides into a right and a 

 left branch, which enter corresponding lungs. The vessel at once 

 divides and subdivides into a number of branches, which, after fol- 

 lowing the bronchial tubes to their termination, give origin to capil- 

 laries that surround the air-cells of the pulmonary lobules. 



The capillaries in this situation are extremely abundant and well 



FIG. 114. DIAGRAM OF THE HEART AND 

 PULMONARY CIRCULATION IN MAMMA- 

 LIANS, a. Right auricle, b. Right ven- 

 tricle, c. Pulmonary artery, d. Lungs. 

 e. Pulmonary vein. /. Left auricle, g. 

 Left ventricle, h. Aorta, i. Vena cava. 

 (Dalton.) 



