xxi] 



CIRCULATORY SYSTEM 



553 



it, so that th& two kinds of blood do not mix in it to any great 

 extent. The opening of the conus is on the right side of the heart 

 (left side as seen from below, Fig. 274), and at the beginning of 

 contraction the venouc blood on the right side of the ventricle 

 flows into the conus. At this stage the conus is relaxed, a con- 

 dition which arranges the longitudinal valve in such a way as to 

 divert the blood almost exclusively into the pulmonary passages, 



Fio. 274. 



A. The heart, after removal from the body, seen from the front, the aortic 

 arches of the left side having been removed. From Howes. 



B. The same from behind, the sinus venosus having been opened up, to show 

 the sinu-auricular valves. 



C. The same, dissected from the front, the ventral wall together with one of 

 the auriculoventricular valves having been removed. 



1. Ventricle. 2. Eight auricle. 3. Left auricle. 4. Truncus arter- 

 5. Carotid arch. 6. Lingual artery. 7. Carotid gland. 

 fotid artery. 9. Systemic arch. 10. Pulmocutaneous arch. 

 mominate vein. ^12. Subclavian vein. 13. Vena cava inferiorr 

 14. Vena cava superior. 15. Opening of sinus venosus into right auricle. 

 Pulmonary vein. 17. Aperture of entry of pulmonary vein. 



Semilunar valves. 19. Longitudinal valve. 20. Point of 



"16. 



18. 



origin of pulmocutaneous arch. 21. Rod passed from ventricle into the 

 truncus arteriosus, indicating the course taken by blood which flows into 

 the carotid and aortic arches. 



whose width and shortness also favours its flow into them. As 

 these become filled the conus contracts, and this has the effect of 

 making the longitudinal valve lie against the openings into the 

 pulmonary arches and so preventing any more blood entering them, 

 while at the same time the path into the systemic arches is widely 

 opened. By this time some of the purified blood received from 



