338 



ZOOLOGY. 



the blood exchanges materials with the tissues (Fig. 32, c.s; 

 c.r) . The capillaries unite to form the smaller veins and these 

 uniting, complete the circuit back to the heart. It is evident 

 that the capillaries are the most important portion of the sys- 

 tem, the part for which the rest in reality exists. 



FIG. 1 66. 



FIG. 167. 



pre. c. 



FIG. 166. Diagram of the heart and branchial arches in Protopterus (one of the 

 Dipnoi). Position and lettering as in the preceding, pre.c., precaval vein, made up 

 of right and left jugulars, subclavians, etc.; post.c., postcaval, made up of the cardinals, 

 right and left. 



Questions on the figure. What are the chief differences between the 

 conditions here and in the preceding figures: (i) as to the heart: (2) as 

 to arteries; (3) as to veins; (4) as to lungs? 



FIG. 167. Diagram of the heart and branchial arches in the Frog, e.g., carotid gland; 

 I, lungs; I. a., left auricle; r.a., right auricle. 



Questions on the figure. How does the heart differ from that of the 

 Dipnoi? How many branchial arches of the aorta are shown? What 

 evidences can you find by comparison that the pulmonary arch is derived 

 from the 3d or 4th branchial? What evidences that the carotid and sys- 

 temic are the first and second respectively? Compare with the table on 

 page 340. Is there anything to indicate that the impurest blood in the 

 heart will go to the lungs? 



