350 MANUAL OF ZOOLOGY sect. 



general body-cavity (abdomen) in which the majority of the 

 internal organs are contained, by a transverse fibrous parti- 

 tion. It consists of four chambers, — the sinus venosus, auri- 

 cle, ventricle, and conns arteriosus. The venous blood enters 

 the sinus venosus from the great veins and passes through 

 the other three chambers in succession in the order given. 

 All the chambers contract rhythmically, and by their con- 

 tractions the blood is propelled from chamber to chamber, 

 and finally driven out from the heart, its passage in the 

 opposite direction being prevented by the presence of 

 valves. These are placed in the openings leading from 

 chamber to chamber, and are so arranged that while they 

 permit the ready passage of the blood in the direction above 

 given, they close up the opening when pressure is exerted in 

 the opposite direction ; thus, for example, when the auricle 

 contracts, the valve guarding the opening leading back into 

 the sinus venosus closes that opening, while the valve in the 

 opening leading into the ventricle opens freely, and the 

 blood passes readily in that direction. The ventricle is by 

 far the most muscular of the four chambers, since it is 

 mainly by its contractions that the blood is forced through 

 the system of vessels. The blood which is forced out from 

 the heart by the contractions of the ventricle passes into a 

 series of vessels which carry it all to the gills. Here it 

 enters a system of capillaries in the gills, and these being 

 separated from the surrounding water only by a thin mem- 

 brane, oxygen readily enters the blood, and the carbon 

 dioxide collected in the various tissues and organs of the 

 body is given off. The blood then enters a set of larger 

 vessels, which combine to form a large trunk, the dorsal 

 aorta. Branches from this distribute blood to all parts of 

 the body, where it enters the systems of capillaries, and 

 whence it is carried back again to the heart by the veins. 



