168 REPTILIA. 



lungs before it returns to the rest of the body, be expressed by a unit, 

 that of Reptiles will be expressed by a fraction of a unit, so much the 

 smaller, as the quantity of blood transmitted to the heart at each con- 

 traction is less. 



As it is from respiration that the blood derives its heat and the fibre 

 its susceptibility of nervous irritation, the blood of reptiles is cold, and 

 the muscular energy less than that of Quadrupeds, and much less than 

 that of Birds ; thus we find their movements usually confined to 

 crawling and swimming ; for, though at certain times several of them 

 jump and run with considerable activity, their habits are generally 

 lazy, their digestion excessively slow, and their sensations obtuse. In 

 cold or temperate climates almost all of them pass the winter in a state 

 of torpor. Their brain, which is proportionally very small, is not so 

 essentially requisite to the exercise of their animal and vital faculties, 

 as to the members of the two first classes ; their sensations seem to be 

 less referred to a common centre, for they continue to live and to 

 exhibit voluntary motions, long after losing their brain, and even after 

 the loss of their head. A communication with the nervous system is 

 also much less necessary to the contraction of their fibres, and their 

 muscles preserve their irritability after being severed from the body 

 much longer than those of the preceding classes ; their heart continues 

 to pulsate for hours after it has been torn away, nor does its loss prevent 

 the body from moving for a long time. 



The smallness of the pulmonary vessels permits Reptiles to suspend 

 the process of respiration without arresting the course of the blood ; 

 thus they dive with more facility, and remain longer under water than 

 either the Mammalia or Birds. 



No Reptile hatches its eggs. The young Batrachians, on quitting 

 the egg, have the form and bronchiee of Fishes, and some of the genera 

 preserve these organs, even after the development of their lungs. 



The quantity of respiration in Reptiles is not fixed like that of the 

 Mammalia and Birds, but varies with the proportion of the diameter 

 of the pulmonary artery compared to that of the aorta. Thus Tor- 

 toises and Lizards respire more than Frogs, &c. ; and hence a much 

 greater difference of sensibility and energy than can exist between one 

 of the Mammalia and another, or between Birds. 



The comparison, however, of their quantity of respiration and of 

 their organs of motion, has enabled M. Brongniart to divide them into 

 four orders, viz. 



The Chelonia, or Tortoises, whose heart has two auricles, and %vhose 

 body, supported by four feet, is enveloped by two plates or bucklers 

 formed by the ribs and sternum. 



