ix THE SKIN 179 



CHAPTER IX 



THE SKIN 



External Characters. — The skin is an organ of unusual 

 importance in the life of the frog, because, in addition to the 

 functions which it commonly performs among other animals, 

 it has a number of special functions which are peculiar to 

 the Amphibia, and which, in most cases, reach their fullest 

 development among the A num. As in most of the Amphibia, 

 the skin of the frog is smooth and moist ; it is very loosely 

 attached to the underlying musculature by thin bands of con- 

 nective tissue, which separate the large subcutaneous lymph 

 spaces. It is everywhere very tough, but it is considerably 

 thicker on the dorsal side of the body than it is below. In 

 certain regions it presents special thickenings ; such as the 

 dermal plicae, which extend backward from near the posterior 

 angles of the eyes, the subarticular pads beneath the joints 

 of the digits of the feet, the swelling at the base of the first 

 finger of the arm, the protuberance over the sixth toe or pre- 

 hallux, and the upper eyelids and lips. Small papillae often 

 occur, especially on the dorsal side of the body, some of 

 which, the tactile papillae, are permanent ; others, the sexual 

 papillae of the female, occur only during the breeding period. 



Histological Structure. — The skin is composed of two 

 principal layers, the epidermis, and the corium, or cutis. A 

 third layer of subcutaneous connective tissue, not belonging 

 to the skin proper, lies underneath the corium and forms the 

 septa uniting the skin to the body wall. 



