264 TEXT-BOOK OF PHYSIOLOGY. 



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-pulmonary 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 

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

 veins (Fig. 115). 



The pulmonary 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. The 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 pulmonary 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-pulmonary air, and the 

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

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

 plished. 



The pulmonary veins which return the 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 pulmonary 

 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 cavse on the right side and the aorta on 

 the left side, by way of the right side of the heart, the cardio-pulmonary 

 vessels, and the left side of the heart. 



The venous blood flowing toward the heart is emptied by the superior 

 and inferior venae cavae into the right auricle, from which it passes through 

 the auriculo-ventricular opening into the right ventricle (Fig. 115); thence 

 into and through the pulmonary artery and its branches to the pulmonary 

 capillaries, where it is arterialized by the exchange of gases the giving up of 

 a portion of carbon dioxid to the lungs and the absorption of oxygen and 

 changed in color from bluish-red to scarlet-red. The arterialized blood, 

 flowing toward the heart, is emptied by the pulmonary veins into the left 

 auricle, from which it passes through the auriculo-ventricular opening into 

 the left ventricle; thence into the aorta and its branches to the systemic 

 capillaries, where it is de-arterialized by a second but opposite exchange of 

 gases the giving up of a portion of its oxygen to the tissues and the absorp- 

 tion of carbon dioxid from the tissues and changed in color from scarlet to 

 bluish-red. The venous blood is again returned by the systemic veins to the 

 venae cavae. Though the blood is thus described as flowing first through the 

 right side and then through the left side, it must be kept in mind that the two 

 sides fill synchronously; that while the blood is flowing into the right side 

 from the venae cavae, it is also flowing from the pulmonary veins into the left 

 side in equal quantities and velocities. 



Though there is but one set of capillaries, as a rule, between arteries and 



