THE FROG 17 



which are under the control of the central nervous 

 system. 



The frog differs much from man and all " beasts," from 

 birds, lizards, snakes, tortoises, and fishes, in the fact that its 

 skin is devoid of hair, feathers, or scales, and there are no 

 claws or nails on the toes and fingers. This, however, whilst 

 true of English frogs, does not hold good for all frogs, nor for 

 the class Amphibia to which the frog belongs. The bull-frog, 

 Ceratophrys, for example, has flat bony plates in the skin of 

 the dorsal surface. 



There are only two median openings on the surface of the 

 body viz. the mouth, a large slit-like opening situated at the 

 anterior end of the head, and the anus or vent (more strictly 

 called the cloacal aperture) which lies at the hinder end of 

 the trunk, between the hinder legs, and nearer to the dorsal 

 than the ventral surface. There is but one set of paired 

 apertures viz. the external nares or nostrils, a pair of small 

 holes placed upon the snout at the anterior end of the head. 

 A bristle passed through one of the nostrils will be found to 

 pass into the cavity of the mouth by an internal nostril. On 

 either side of the head is an eye, which has an eyeball, a 

 coloured portion, or iris, and a pupil, as in ourselves. Each 

 eye is protected by two eyelids, of which the upper is thick, 

 pigmented like the rest of the skin, and nearly immovable; 

 the lower is thin, semi-transparent, and freely movable. 



Just behind the eye, on either side of the head, is a patch of 

 dark colour, in the middle of which a circular membranous 

 area, bounded by a firm, somewhat raised, ring, can be seen. 

 This is the tympanic membrane, or drum of the ear. The 

 frog has no external valve of the ear as have man and beasts, 

 nor has it a passage leading down from the exterior to the 

 ear-drum ; the latter is on the surface. 



The frog's limbs have a general correspondence with our 

 own, as may be seen at a glance. The front limb or arm is 

 divisible into an upper-arm, technically called the brachium, a 

 fore-arm or ante-brachium, and a hand or manus. The wrist, or 

 carpus, is hardly distinguishable externally, but we shall learn 

 more of it when we study the skeleton. 



The hind limb comprises a thigh or femur, a leg or crus, a 

 much elongated ankle or tarsus, and a foot or pes. 



On the hand there are four fingers or digits corresponding 



