6o THE COMMON FROG. [chap. 



CHAPTER VI. 



THE SKELETON OF THE FROG. 



It may cause surprise to speak of the skin of the 

 common Frog as part of its skeleton, consisting as 

 the skin does of small membranous structures only. 



The term " skeleton," however, should properly in- 

 clude all the membranous and gristly, as well as the 

 bony structures.^ Moreover, more or less of the skin 

 may attain to so solid a condition as to justify its 

 comprehension under the name " skeleton," even in 

 the popular signification of that term. 



The skin of Vertebrate animals consists of two 

 layers : an outer layer (the epidermis or ecteron), and 

 an inner layer (the dermis or enderon). The epidermis, 

 and any projections or processes developed from it 

 when they take on a dense or hardened structure, 

 become horny. Of such horny nature are hairs, 

 feathers, nails, and scales ; they are more or less 

 epidermal appendages. The dermis when hardened 

 becomes bony, and of such nature are the bony skin- 

 plates or " scutes " and teeth. They are dermal 



* See " T^essons in Elementary Anatomy," Lesson II., p. 22. 



