CIRCULATION IN THE AMPHIBIA AND FISHES. 133 



Fig. 84. 



sends a branch to r. The arrows show the course of 

 the circulation. The blood flows into the auricle d, 

 from the vein c, which comes from all parts of the 

 body, except the lungs. It also flows through the vein 

 m, which comes from the lungs, and is called the pulmo- 

 nary vein. Both these vessels 

 deliver the blood in a continued 

 stream. When the auricle is full 

 of blood, it contracts and throws 

 its contents into the ventricle e, 

 The ventricle then contracts and 

 sends a part of the blood through 

 the pulmonary artery n, to the 

 \\a lungs, while the other, and greater 

 | portion passes through the aorta, 

 a, to all the other parts of the 

 system. While the blood is thus 

 flowing through the large vessels, 

 near the heart, it is constantly 

 passing from the small arteries 

 into the small veins, which com- 

 municate with each other in 

 every part of the body, as shown 

 at b and r. These little veins form branches which 

 grow larger, as they approach the heart, as those of the 

 arteries grow smaller as they recede from it, in the 

 same manner that the limbs of a tree are enlarged as 

 they approach the root, and lessened towards the top. 

 The circulating fluid being thus collected from all parts 

 of the system, is constantly pouring into the auricle by 

 the great veins, to be again sent to all its parts, by the 

 ventricle, and so on, in a continued round during the life 

 of the animal. 



This simple, or single apparatus is not only such as is 

 found in the Frog, but may be taken as an example of 

 the circulating system in all the cold blooded quadru- 

 peds. 



Explain the circulation as it occurs in the most simple form, pointing 

 out the names of the different parts of the apparatus, and showing th 

 course of the blood? To what class of animals does this simple circu- 

 lation apply ? 



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