AND PHYSIOLOGY. 



233 



additional impulse to the blood as it is forced into the pulmo- 

 nary arteries The need of this arrangement is to give a full 

 supply of blood to the lungs, which the ordinary powers of 

 the heart could not effect. 



Fig. 231. 



Arterial System of a Bird. 

 A-teries of the Grebe, a, Aorta, am, One of its large Branches. It gives off the 

 Carotid (ac) and Subclavian, is ultimately distributed to the muscles of the chest, and 

 corresponds to the mammary arteries of mammals, ad, One of the Branches of the 

 Vertebral Artery supplying the Muscles of the Shoulder, ce, Arterial Loops formed by 

 the Branches of the External Carotid, al, Lingual Artery, t, Trachea, or wind-pipe. 

 ar, Renal Arteries, ai, Ischiatic Artery proceeding to the lower extremities, as, Sac- 

 ral Artery, forming a continuation of the Aorta, and giving origin to the Inferior Mesen- 

 teric Artery, etc. c/, The Cloaca. 



Why do the valves have an especial muscle ? 



