92 RESPIRATION. 



and tortoises, the air is swallowed. Respiration in these ani- 

 mals is not performed so regularly and constantly as in the 

 higher classes. Only a comparatively small proportion of the 

 blood is subjected to the influence of the air, at once ; and 

 they can subsist for a very considerable time without breath- 

 ing, though its suspension at length destroys them. Tortoises 

 have been known to live more than a month with their jaws 

 tied closely together, and their nostrils filled with sealing- 

 wax. A toad lived for five days in a jar containing about a 

 hundred cubic inches of air. In forty inches, another toad 

 lived for twenty-four hours, and a frog for fifty-nine. This is 

 many times longer than a warm-blooded animal could exist 

 under the same circumstances. 



' The temperature of the bodies of Reptiles is generally 

 that of the air and water in which they are found. Still they 

 have the power of resisting, during life, both, very high and 

 very low temperatures; and as their heat is seldom, under 

 any circumstances, raised to a degree near to that of our 

 bodies, they are designated as cold-blooded animals. This 

 circumstance proceeds, probably, in some way from the limited 

 quantity of their respiration ; and with the same cause is con- 

 nected their slow and feeble motions, their tendency to the 

 dormant state, and in jjeneral their low degree of vital power. 



' The Respiration of Fishes is carried on by means of gills 

 or branchiae, to which the air is applied through the medium 

 of the water. Every portion of water contains a certain quan- 

 tity of air combined or mixed in some way with it, and by 

 this means is made capable of supporting respiration. A cur- 

 rent of water is constantly passed over the gills by the action 

 of the mouth, and produces the requisite change upon the 

 blood circulating through them. This change is of the same 

 kind with that taking place in the warm-blooded animals. It 

 arises from the influence of the oxygen in the atmospheric 

 air ; and if the water be examined, after fishes have respired 

 it, the air it contains will be found to have undergone a simi- 

 lar change of composition with that breathed by quadrupeds 

 and birds.' 



When a free communication with the external air is pre- 

 vented by ice, or by artifice, fishes immediately discover symp- 

 toms of uneasiness, and soon perish. ^Elian informs us, that, 

 in winter, when the river Ister was frozen, the fishers dug 

 holes in the ice ; that great numbers of fishes resorted to 

 these holes ; and that their eagerness was so great, that they 

 allowed themselves to be seized by the hands of the fishermen 



