266 . TEXT-BOOK OF PHYSIOLOGY 



of the lungs, the pulmonic veins. Below is found the corresponding auri- 

 culo-ventricular opening. The wall of the auricle measures about 3 mm. 

 in thickness. The left ventricle is conic in shape from above downward 

 and oval or circular in shape on cross-section. At its upper inner 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-developed 

 system of columnae carneae, musculi papillares, chordae tendineae, etc. The 

 wall of the left ventricle measures about 11.5 mm. in thickness 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 endocardium. It contains also elastic fibers 

 and smooth muscle-fibers. Its entire surface is covered with a layer of 

 polygonal endothelial cells. This membrane serves partially to resist undue 

 distention of the heart during contraction and to prevent separation of the 

 muscle-fibers. The endocardium is continuous with the lining membrane 

 of the blood-vessels. 



The inter-auricular septum is quite thin and composed of the two layers 

 of the endocardium, between which is a layer of muscle-fibers. It presents 

 at its lower portion an oval depression, the fossa ovalis. 



The inter- ventricular septum is quite thick and well developed, and com- 

 posed of the two layers of the endocardium enclosing the muscle-fibers. In 

 the upper and central portion of the septum, there is, however, a small region 

 which is thin owing to the absence of muscle-tissue and composed of endo- 

 cardium only. This region is known as the pars membranacea septi. 



The Cardio-pulmonic Vessels. Though the two sides of the heart 

 are separated from each other by the auriculo-ventricular septum, they are 

 anatomically and physiologically connected by the intermediation of the 

 pulmonic system of vessels: viz., the pulmonic artery, capillaries, and 

 veins (Fig. 109). 



The pulmonic artery arises from the conus arteriosus of the right ven- 

 tricle. After a short upward course it divides into a right and a left branch, 

 which enter the corresponding lungs. Each vessel at once divides and sub- 

 divides into a number of branches, which, after following the bronchial tubes 

 to their termination, give origin to capillaries that surround the air-cells of 

 the pulmonic lobules. 



The capillaries in this situation are extremely abundant and well developed. 

 They lie close to the inner surfaces of the air-cells. The blood is thus 

 brought into intimate relationship with the intra-pulmonic air, and the 

 exchange of gases the excretion of carbon dioxid and the absorption of 

 oxygen for which the cardio-pulmonic vessels exist, is readily accom- 

 plished. 



The pulmonic veins which return the arterialized blood to the heart are 

 formed by the convergence and union of the small veins which emerge 

 from the capillary system. The final trunks thus formed, the four pulmonic 

 veins two from each lung enter the posterior wall of the left auricle. 



The Course of the Blood through the Heart. There is thus estab- 

 lished a pathway between the venae cavae on the right side and the aorta on 



