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the members of the sloth, lory, and other tardi- 

 grade animals, as they are called, the slow pace 

 of which renders a supply of blood with the usual 

 impulse unnecessary. With respect to the veins, 

 also, there is a peculiarity in the large trunks of 

 this system in the seal, sea-otter and other diving 

 quadrupeds, which merits attention, as display- 

 ing a beautiful example of adaptation in the 

 structure of organs to the habits of the animal in 

 which they are found. From what has been said 

 under the head of the circulation of fishes, it will 

 be obvious that that of quadrupeds must be im- 

 mediately stopped when the lungs cease to act ; 

 since the passage of the blood from the right side 

 of the heart to the left can take place only through 

 these organs. When, therefore, an animal of 

 this description is under water, the blood has a 

 tendency to accumulate in the lungs, which can 

 no longer transmit it forwards, and would rapid- 

 ly produce suffocation, if it were not, by some 

 means or other, kept from them. Now, nature 

 has given to the animals just mentioned, as well 

 as to others habitually exposed to the same impe- 

 diment of the respiratory process, a large pouch 

 in the course of their chief vein, in which the 

 blood, hurrying towards the right cavity of the 

 heart and the lungs, becomes collected during 

 the suspended action of the latter ; and suffoca- 

 tion is thus obviated, till, on the ascent of the 

 animal again to the surface of the water, the 

 blood freely repasses through its natural channels. 



