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 auditus ; D, os hyoides: £, cervical vertebre 3 
F, dorsal vertebra ; G, sacrum ; H, caudal vertebra ; 1, dorsal ribs; K, marginal scales ; N, scapula; 
0, coracoid bone ; Pp, os humeri; Q, radius; R, ulna; s, bones of the carpus ; T, metacarpal bones ; 
u, digital phalanges; v, pelvis; w, femur; x, tibia; y, fibula; z, tarsus; ®, metatarsus; 
a.v., 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 
Jeft half, which receives arterial or aérated 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 
