166 MANUAL OF ANIMAL BIOLOGY 



to the eyes, a pair of large, circular tympanic membranes are 

 conspicuous ; one on each side of the head. These are membra- 

 nous, disc-like structures embedded in the skin which function as a 

 part of the auditory apparatus, or ear. They are, in fact, ear 

 drums, essentially the same as in Man, but lying on the surface. 



Attached to the trunk are two pairs of jointed pentadactyl 

 limbs which are adapted both for aquatic and terrestrial life. 

 These appendages represent a tremendous modification of, and 

 advance over, the paired fish fin, and the various constituent 

 structures are homologous with those in the limbs of the higher 

 classes of Vertebrates. 



The fore limb is composed of three main parts, commonly known 

 as the upper arm, forearm, and hand. The latter is composed 

 of the wrist and four well-formed digits, together with a very rudi- 

 mentary first digit, corresponding to our thumb, which can be felt 

 through the skin. The hind limb has the same general structure 

 as the fore limb, but is much larger and more powerful. The 

 various parts of it are commonly spoken of as the thigh, shank, 

 and foot. The latter is composed of the ankle and five complete 

 digits. Between the digits an interdigital web is developed. 

 A more detailed description of the bones of the limbs and also of 

 the general skeletal structure will be given later. 



The outer covering of the Frog's body consists of a smooth slimy 

 skin which is heavily pigmented, particularly on the dorsal surface 

 of the body. The skin of the Frog and of other Vertebrates is a 

 very important structure. It is far more than a mere external 

 covering. In general, it may be said to have four important func- 

 tions, namely, support, protection, respiration, and secretion. In 

 connection with the function of support, it is apparent that the 

 skin is essentially a heavy, resistant covering enclosing the various 

 body tissues. 



The skin functions beautifully as a protection against unfavor- 

 able environmental features. In the first place there are very 

 few disease germs that can penetrate the unbroken skin. Also 

 the skin is the tissue in which the various protective exoskeletal 

 features are formed, such as the scales of Fishes and Reptiles, the 

 feathers of Birds, the hair of Mammals, as well as claws, nails, and 

 hoofs. The Frog's skin entirely lacks these hard exoskeletal 

 structures, but they are of general occurrence in most Vertebrates. 

 Another feature which is well-shown in the skin is protective 



