188 NATURAL HISTORY. 
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 the parts of the skeleton are absent ex- 
cept the head, the chain of vertebre, 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 to 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 
