4 LECTURE VII. 



subdivisions, and exhibit the artery and vein in 

 a more simple state of ramification than in the 

 Frogs. In the Serpent tribe the structure of the 

 lungs seems to run between that of Frogs and 

 Lizards ; the upper part being divided internally 

 into smaller cells, while the remainder degenerates 

 into a mere continued bladder as in the Lizards. 



The whole tribe of the proper or Linnaean 

 Amphibia, viz. the Tortoises, Frogs, Lizards, and 

 Serpents, possess a kind of voluntary power by 

 which they are able at pleasure to suspend their 

 respiration; so as to continue for a long time 

 without breathing, by retaining air within their 

 lungs, instead of being obliged to discharge it 

 frequently, as quadrupeds and birds are obliged to 

 do. Linnaeus therefore, among other characters 

 of the Amphibia gives that of arbitrary lungs, 

 pulmones arbitrarily or such as can at pleasure 

 suspend respiration, without injuring the animal. 

 Upon this principle it is that these animals may 

 be confined in the closest situations without seem- 

 ing to suffer any material inconvenience, and 

 many of them are calculated for residing with al- 

 most equal ease either on land or under water. 

 In Frogs so strong is this power of retaining air 



