258 



THE MECHANICS OF THE HEART 



the aerated, the former being held in the compartment to the right^ 

 and the latter largely in the space to the left of this septimi. During 

 the contraction of the ventricle, the edges of the septal flaps are brought 

 together so that the largest amount of the venous blood is forced into 

 the pulmonary artery, while the oxygenated blood is diverted chiefly 

 into the aorta. But while definite provision has been made in these 

 animals to prevent a complete mixture of the venous with the aerated 

 blood, a certain degree of intermingling is still possible in several places 

 outside the heart. Excepting certain fish, the reptilian heart is the 

 first to exhibit a system of blood-vessels for the nutrition of the cardiac 



musculature. The hepatic portal is 

 associated with a renal portal system. 

 The heart of birds possesses four 

 chambers, namely two auricles and two 

 ventricles. A distinct vestibular por- 

 tion is not present. The blood is re- 

 turned from the tissues by the right 

 and left post, cavae. It enters the right 

 auricle and then the right ventricle, 

 whence it is conveyed to the lungs 

 through the pulmonary artery. Four 

 pulmonary veins conduct it from here 

 to the left auricle and left ventricle, 

 whence it again attains the peripheral 

 tissues by way of the aorta and its 

 branches. Thus, for the first time, the 

 aerated blood is completely separated 

 from the venous blood by a longitudinal 

 septum which divides the heart into a 

 right and a left side. Each side in 

 turn embraces an antechamber, or 

 auricle, and a main chamber, or ven- 

 tricle. The auriculoventricular orifices 

 are guarded by membranous valve 

 flaps, the right being large and muscular. The aortic and pulmonary 

 orifices are beset with three cup-shaped valve-flaps. Owing to the 

 functional importance of the wings and the corresponding massive- 

 ness of the pectoral muscles, the arteries supplying these parts are very 

 large in caliber. Moreover, in agreement with the position of the legs, 

 the femoral blood-vessels are found far forward in the body. 



The Circulatory System in Mammals. — In mammals, the heart 

 is divided into a right and left half and each half in turn into an ante- 

 chamber, or auricle, and a main chamber, or ventricle. The blood 

 which is returned from the tissues, enters the right auricle by way of 

 the superior and inferior venae cavae, while the blood from the lungs 

 is conducted into the left auricle by way of the pulmonary veins. 

 Two distributing channels leave the heart, namely, the pulmonary 



Fig. 127. — Diagram to Show 

 THE Course of the Blood Through 

 THE Heart of Birds. 



PC, post, cavae; /2A, right auricle; 

 LA, left auricle; RV, right ventricle; 

 LV, left ventricle; PA, pulmonary 

 artery; PV, pulmonary vein; A, 

 aorta. 



