COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 247 



attached by delicate cords, the chordae tendinae, to the wall of the ventricle. 

 The wall of the ventricle has several muscular ridges which project into the 

 cavity; they are called the columnae carnae. Find, to the medial side of the 

 mitral valve, the opening of the left ventricle into the aorta. Probe into this 

 and satisfy yourself that it leads into the aorta. Note the three semilunar valves 

 at the beginning of the aorta. 



The removal of the heart permits the tracing of the esophagus into the pro- 

 ventriculus and of the bronchi into the lungs. The form and extent of the lungs 

 can also be observed to advantage at this time. 



7. The circulation through the heart and the comparison of the circulatory system of 

 bird and reptile. In birds the heart is completely divided into right and left auricles and 

 ventricles. The venous blood enters the right auricle from the systemic veins, passes into the 

 right ventricle, and out into the pulmonary arteries which convey it to the lungs. After 

 aeration in the lungs the blood returns by way of the pulmonary veins to the left auricle, from 

 which it flows into the left ventricle and out of the aorta. Thus, the right side of the heart 

 contains only venous blood and the left side only arterial blood. There is a perfect double 

 circulation, both kinds of blood flowing simultaneously through the heart, the two streams 

 completely separated from each other. Since the aorta is connected only with left ventricle, 

 the arterial system receives pure arterial blood. The perfection of the double circulation is 

 achieved by the splitting of the ventral aorta into two trunks aorta and pulmonary which 

 are connected with the left and right sides of the heart, respectively. The sinus venosus is 

 apparently absent, but in reality is reduced and incorporated into the right auricle. The 

 conus arteriosus is represented by the semilunar valves of the aorta and pulmonary artery. 



The aortic arches are further modified from the condition seen in reptiles. As in the 

 turtle, the bases of the common carotid arteries represent the third aortic arches. The union 

 of the subclavians with these is secondary. The arch of the aorta is the right fourth aortic 

 arch, the left fourth arch having vanished during embryonic development. There is con- 

 sequently in birds no complete aortic arch as in the preceding forms but only, so to speak, 

 one-half an arch, the persistent half being the right one. The pulmonary arteries represent 

 the sixth aortic arch, separated as in reptiles from the aorta (Fig. 58^, p. 267). 



The venous system is reptilian in character. The two precaval veins are similar to those 

 of the turtle and are homologous with the anterior cardinal veins of lower forms (the internal 

 jugular branch being the original anterior cardinal). The bases of the precavals entering the 

 sinus venosus are the common cardinal veins. The posterior cardinal veins are as in the 

 turtle represented only by their posterior portions, which are named in the adult the renal 

 portal veins. These veins have, as in reptiles and Amphibia, absorbed the veins of the legs 

 and tail. In birds it is very interesting to note the union which is in progress between the 

 renal portal system (posterior cardinals) and the postcaval vein. As we shall see, this union 

 is complete in mammals. In birds the renal portal system is probably to a slight degree 

 functional as a portal system through the kidneys, blood passing from the renal portal vein 

 into the kidneys and re-collecting into the renal veins tributary to the postcaval vein. Most 

 of the blood, however, passes directly from the renal portal veins into the postcaval. 



The origin of the postcaval vein in birds is the same as that given for reptiles. That 

 part of it which is situated between the kidneys is formed of the subcardinal veins, chiefly the 

 right one. The part through and anterior to the liver comes from the vitelline veins. The 

 middle region of the vein is a new formation. The postcaval is seen to be usurping the renal 

 portal system and thus extending itself posteriorly. This process is completed in mammals. 



