CIRCULATION IN MAMMALS. 



319 



mitted by the hepatic vein to the vena cava. Thus that portion of 

 blood -which supplies the liver with the materials of its secreting action, 

 passes through two sets of capillaries, between the time of its leaving 



Fig. 92. 



latomy of the Human Heart and Lungs. 1. The right ventricle; the vessels to the right of the figure 

 the middle coronary artery and veins ; and those to its left, the anterior coronary artery and veins. 

 2. The left ventricle. 3. The right auricle. 4. The left auricle. 5. The pulmonary artery. 6. The right 

 pulmonary artery. 7. The left pulmonary artery. 8. The remains of the ductus arteriosus. 9. The arch 

 of the aorta. 10. The superior vena cava. 11. The right arteria innominata, and in front of it the vena 

 innominata. 12. The right subclavian vein, and behind it its corresponding artery. 13. The right common 

 carotid artery and vein. 14. The left vena innominata. 15. The left carotid artery and vein. 16. The left 

 subclavian vein and artery. 17. The trachea. 18. The right bronchus. 19. The left bronchus. 20, 20. 

 The pulmonary veins; 18, 20, form the root of the right lung; and 7, 19, 20, the root of the left. 21. The 

 superior lobe of the right lung. 22. Its middle lobe. 23. Its inferior lobe, 24. The superior lobe of the 

 left lung. 25. Its inferior lobe. 



the heart and its return to it. The portal circulation in Birds, as in 

 Reptiles and Fishes, receives the blood from the posterior part of the 

 body, and from the extremities ; but the portal blood is only conveyed 

 to the liver ; the kidneys being supplied by the renal artery. 



566. This perfect form of the Circulating apparatus is only attained, 

 in the warm-blooded animal, after a series of transformations, which 

 strongly remind us of the permanent forms presented by the vascular 

 system in Fishes and Reptiles. Thus in the embryo of the Chick at 

 about the 60th hour, and in that of the Dog at about the 21st day, 

 the curved and dilated tube, of which the heart previously consisted 

 ( 554), is found to be distinctly divided into an auricle and a ven- 

 tricle. From the latter originates the main arterial trunk, which 

 divides into four pairs of lateral branches ; and these pass round the 

 pharynx, precisely in the position and direction of the arteries of the 

 gills of Fishes. They do not, however, distribute the blood to gill- 

 tufts ; for none such are developed in the embryo of the warm-blooded 

 animal: but they meet again below the pharynx, to form a trunk, 

 which supplies the general circulation. Within a short period, how- 

 ever, the whole plan of the circulation undergoes a change. The auricle 





