STRUCTURE OF THE VERTEBRATES 237 



the left ventricle; (2) the mammal embryo retains the primitive 

 connection between the pulmonary artery and the dorsal aorta 

 (the distal portion of the sixth arch) and this tends to shunt 

 the blood away from the lungs; and (3) the pressure within the 

 collapsed lungs prevents the blood from entering. The connecting 

 vessel between the pulmonary artery and the aorta is called the 

 ductus arteriosus (or ductus Botalli). It begins to degenerate 

 soon after birth, but may remain open for some months in the 

 longer lived animals. Breathing is initiated in the newborn mam- 

 mal by a hypertension of carbon dioxide in the blood, and the 

 ductus arteriosus degenerates in correlation with the function 

 of the lungs. 



. The cardiac cycle depends upon the alternate contraction and 

 relaxation of the heart and the flow of blood in one direction 

 through the arteries and veins. The latter is made possible by the 

 system of valves which direct the flow through the heart. These 

 are: (1) the auriculo-ventricular valves separating the auricles 

 from the ventricles; and (2) the semi-lunar valves at the base 

 of the aorta and the pulmonary artery. 



Assume that the heart muscles are completely relaxed. The 

 blood then pours into the auricles from the systemic veins and 

 passes directly into the ventricles. As the ventricles fill, a wave 

 of contraction begins in the auricles, in the node of tissue formed 

 by the primitive sinus venosus. This wave spreads downward, 

 continuing into the ventricles. The swirl of blood in the ventricles 

 has thrown the valves outward, and the ventricular contraction 

 forces them upward and together, preventing the flow of blood 

 backwards into the auricles. The continued contraction of the 

 ventricles forces the blood into the aorta and pulmonary artery. 

 Relaxation occurs in the same order as does the contractile wave, 

 and as the pressure in the ventricle decreases the semi-lunar 

 valves are forced open by the column of blood in the arteries. 

 The fluid is unable to flow backward into the ventricle, and 

 is forced into the smaller arteries by a peristaltic wave of con- 

 traction in the larger vessels which carries it into the capillaries. 

 The heart completes its relaxation, and the cycle begins anew. 

 The closure of the valves is due to a differential pressure 

 within the regions of the heart and arteries. The sounds of the 

 heart are made by the snap of the valves as they close, the 



