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 



B 



Digitized by Microsoft® 



