174 



THE INHABITANTS OF THE SEA. 



long time without breathing ; long enough, at least, to tire the 

 patience of the most obstinate foe. 



Skeleton of Tortoise. 

 a, superior maxilla ; b, inferior maxilla ; c, ossiculum audltus ; d, os hyoides : e, cervical vertebra? ; 

 p, dorsal vertebra; ; G, sacrum ; u, caudal vertebrae ; I, dorsal ribs ; K, marginal scales ; N, scapula ; 

 o, coracoid bone ; p, os humeri ; q, radius ; 11, ulna ; b, bones of the carpus ; T, metacarpal bones ; 

 J, digital phalanges ; V, pelvis ; \v, femur ; x, tibia ; Y, fibula ; z, tarsus ; A, metatarsus ; 

 i.v. f phalanges of the foot. 



But how comes it, the reader may ask, that respiration, which 

 pours a warm current through our veins, fails in raising the 

 temperature of the turtle's blood ? 



Without entering into a lengthened description of the human 

 heart, I shall merely observe that it consists of two halves (each 

 half being again subdivided into two separate chambers), and 

 that the right half, which receives venous blood and pours it 

 into the lungs, is completely separated by a partition from the 

 left half, which receives arterial or aerated blood from the lungs, 

 and propels it into every part of the body. Thus the two dif- 

 ferent kinds of blood are completely separated, so that an 

 unmixed venous blood flows into the lungs, where it is converted 

 by the oxygen of the air into arterial blood. But this connection, 

 like most chemical processes, takes place under an evolution of 



