io8 The Organs of Respiration 



waste of tissues, after which the blood again passes to the heart 

 through the cardinal vein. From the intestine the blood, charged 

 with nutritive materials in solution, is carried by the portal vein 

 to the liver. Here it again passes by the hepatic sinus to the 

 sinus venosus and the heart. 



The details of the circulatory system vary a good deal in the 

 different groups, and a comparative study of the direction of 

 veins and arteries is instructive and interesting. 



The movement of the blood in fishes is relatively slow, and 

 its temperature is raised but little above that of the surround- 

 ing water. 



