188 NATURAL HISTOEY. 



exposures to a wide range of temperature in the atmos- 

 phere. Thus, in man, the natural degree is 98 by Fahr- 

 enheit's thermometer, many degrees above ordinary sum- 

 mer's heat. This degree is maintained even in the severe 

 cold of the arctic regions. There are various expedients 

 for keeping in the heat made in the blood of the warm- 

 blooded Vertebrates. Hair and fur do it in quadrupeds, 

 feathers in birds, and blubber in whales. Man does the 

 same by making for himself garments of materials which 

 are good non-conductors of heat. .Now in the cold-blood- 

 ed division there is less heat made in the blood, and their 

 coverings are not calculated to retain it. These animals, 

 therefore, have a tendency to take the temperature of 

 the air or water with which they are surrounded. 



303. I will first speak of Reptiles. These are so called 

 from the Latin word repto, to creep or crawl ; for, al- 

 though some of this class have four feet, their limbs are 

 generally so short that a portion of the body is dragged 

 along upon the earth. 



304. The skeleton is much more varied in Reptiles 

 than in the warm-blooded Vertebrates. In some of the 

 Snake group all ibtie parts of the skeleton are absent ex- 

 cept the head, the chain of vertebrae, and the ribs, which 

 are very numerous, amounting, in some cases, to several 

 hundred. While in the Snake tribe the breast-bone is 

 wanting, in the Turtle tribe it is expanded into a large 

 under shield, the ribs also expanding above into an upper 

 shield. 



305. Reptiles can execute less rapid and less prolonged 

 motions than warm-blooded animals. This is because the 

 blood which circulates in the muscles and in all their or- 

 gans is less stimulating. This can be seen te be true by 

 observing how the mode of their circulation differs from 

 that of Mammals and Birds. That this may be clear to 

 you, I will make use of two diagrams, showing the plan 

 of the circulation in each. 



306. The first diagram, Fig. 155, giving the plan of the 



