CIRCULATION AND RESPIRATION 127 



the left atrium. The plan of the blood circulatory system of the 

 frog provides for two circuits, a general systemic circulation and a 

 pulmonary circuit, but these two circuits are not completely 

 separated because they both make use of a single ventricle in 

 passing through the heart. 



Action of the Frog's Heart. — The study of the living heart 

 shows that each atrium and the ventricle contract and dilate in a 

 regular sequence. Contraction of a part of the heart is known 

 as systole, and the dilatation which follows is diastole. The order 

 of sequence of contraction is right atrium, left atrium, ventricle. 

 Immediately after the ventricular systole, the bulbus cordis 

 swells and then contracts to its normal size. Since both atria 

 contract before the ventricle, the latter becomes filled with 

 venous blood from the right atrium and oxygenated blood from the 

 left. However, owing to the spongy internal structure of the 

 ventricle and to the fact that ventricular systole follows almost 

 immediately after the contraction of the left atrium, there is 

 probably little mingling of the two kinds of blood in the ventricle. 

 When the ventricle contracts, the blood is forced into the bulbus 

 cordis, the atrioventricular valves preventing backflow of blood 

 into the atria. The first blood to leave the ventricle is the 

 venous blood since it occupies a position on the right side of the 

 heart near the entrance to the bulbus cordis. This venous 

 blood seems to enter the cavum pulmocutaneum and thence 

 flows into the pulmocutaneous arteries for two reasons, viz., (1) 

 the relatively slight peripheral resistance in the pulmocutaneous 

 trunks as compared with the pressure in the carotid and systemic 

 arches, and (2) the contraction of the bulbus brings the free edge 

 of the bulbar septum in contact with the bulbar wall, closing the 

 cavum pulmocutaneum immediately after it is filled. Backflow 

 into the bulbus is prevented by the anterior bulbar valves. The 

 opening into the pulmocutaneous arteries is guarded by valve 

 16 on the dorsal side of the septum bulbi and by valve 3 which 

 lies opposite on the wall of the bulbus. The aortic opening is 

 guarded by valve la, on the ventral side of the septum bulbi, 

 and by the large valve 2, which is attached to the ventral wall 

 of the bulbus. In each aortic arch, just before turning poste- 

 riorly, are semilunar valves. Each carotid artery is provided 

 with a carotid gland at the point of bifurcation into internal and 



