1 8 COMPARATIVE ANATOMY 



to the four fingers of our own hand. A rudiment of a thumb 

 is present, but it is very small and hidden under the skin. 

 Anatomists count the fingers from thumb to little finger, and 

 sometimes distinguish them by the following names : i. Pollex, 

 2. Index, 3. Medius, 4. Annularis, 5. Minimus. In the 

 male frog, the index develops a rough, cushion-like swelling in 

 the breeding season. The fingers of the frog are not webbed. 

 Though the foot has only five apparent toes a close inspection 

 reveals the presence of six. The first, counting from the 

 inner side, is extremely small, and almost hidden beneath the 

 skin. It is a moot point whether it should be regarded as the 

 representative of the great toe or hallux of man, or whether it 

 should be regarded as an accessory toe, not represented in man. 

 It is usual to consider the toe next to it, the shortest of the 

 remaining five, as the great toe. Of the five apparent toes the 

 fourth is the longest : they are united by a membranous ex- 

 pansion, and are said to be webbed. 



The frog's mouth is furnished with distinct jaws ; the upper 

 jaw is a forward continuation of the head, and is not in- 

 dependently movable ; the lower jaw is hinged on to the 

 posterior region of the head, and is freely movable in a 

 vertical direction. Jaws of this kind, opening vertically, are 

 only found in vertebrates that is, in animals which have a 

 back-bone, and in them only in the division (which comprises 

 nearly the whole group) known as the Gnathostomata. The 

 mouth is furnished with fine pointed teeth arranged in a single 

 row on the edges of the upper jaw, and there are also two 

 small patches of teeth on the fore-part of the roof of the 

 mouth, called vomerine teeth. There are no teeth on the 

 lower jaw. The toad, so like the frog in most respects, has 

 no marginal teeth on the upper jaw. On opening the jaws 

 the wide buccal cavity is seen, which narrows posteriorly to 

 form the gullet or oesophagus. Notice (i) the posterior nares, 

 to the outside of and in front of the patches of vomerine 

 teeth; (2) the Eustachian tubes, a pair of relatively large 

 apertures, one on each side of the hinder part of the buccal 

 cavity : each leads into a cavity, the tympanic cavity, which 

 is closed externally by the tympanic membrane ; (3) the 

 glottis, a slit-like aperture on the floor of the hinder part of 

 the buccal cavity : it leads through a short larynx into the 

 lungs ; (4) the prominences on the roof of the buccal cavity, 



