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 con- 

 tinuation of the head, and is not independently 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, formed by the projection of the lower sides of the eye- 



