274 AMPHIBIA 



additional rudimentary finger which is easily felt under the skin. 

 The two inner fingers contain three joints each and the two outer 

 four joints each. The " thumbs " of the male are thickened notice- 

 ably. The hind legs are long and well suited for jumping and swim- 

 ming but of little service in walking. They are also divided into 

 three parts, the thigh, shank, and foot. The foot is well developed 

 and has the ankle remarkably elongated. There are five toes and 

 the rudiment of a sixth toe called the prehallux^ situated on the 

 inner side of the foot. 



The toes increase in length from the first to the fourth. The 

 fifth is a little shorter than the third. The first two toes contain 

 three joints each, the third and fourth contain four each, and the 

 elongated fourth toe has five joints. The webbed hind feet are 

 used in swimming. 



The Skin and Its Appendages. — The loose skin consists of the 

 cuticle or epidermis and the cutis vera, corium or dermis, and 

 contains numerous glands. The epidermis has several layers of 

 epithelial cells, the outer ones horny and flat, the middle polygonal, 

 and the inner columnar. Each cell has a distinct nucleus. In 

 the deeper cells the nucleus is broad, oval and rounded; in the other 

 cells it is flattened and thin. The horny layer is usually very thin, 

 consisting of one or two layers of flattened cells, but on the back 

 and on the under side of the toes it is very thick and rough. Con- 

 tractile ^/o^w^w/ cells are also found in the epidermis. Just beneath 

 the horny layers are found the goblet cells or mucous cells. These 

 are supposed to contain a substance important in connection with 

 the process of casting oflF the skin. Toads and frogs are known to 

 eat their shed skins. 



Cutis Vera. — The epidermis is attached to the cutis by means of a 

 continuous layer of branched cells deeply stained when the ani- 

 mals are fed certain foods. Many of these cells are pigmented. 

 This layer is seldom flat, but raised into papillae or folds which are 

 repeated by the superimposed epidermis. In addition to this layer 

 the corium (dermis) has, except in webs and supplemental toes, three 

 distinct layers of connective tissue, with much unstriped muscle 

 fiber. 



The skin is loose In structure and serves for an important lymph 

 space; accordingly the frog is much used in medical studies oi edema. 

 The muscle ^^«- of the cutis is unevenly distributed. It is found in 

 the back of the dorsal surface of the head and neck and less freely 



