i82 POPULAR SCIENCE MONTHLY. 



history, he will be surprised to find how wide many of the latter 

 are of the mark with respect to the attitude the animal really 

 assumes in Nature. 



When a frog is at rest in this sitting position he presents us 

 with a number of external characters that are very interesting to 

 study. It will be noticed that the little apertures forming the 

 nostrils open and shut alternately, while at the same time the 

 mouth is closed, and the rising and falling of the skin covering the 

 throat show that a pumping operation is going on. This is just 

 exactly what is taking place, and the air pouring in through the 

 nostrils has to be swallowed in order to be conveyed to the lungs. 

 There being no ribs, the chest can not enter into this respiratory 

 act, so a frog can be easily suffocated by prying its mouth open 

 for a time. The skin in these creatures also forms a very impor- 

 tant part of the respiratory apparatus, and a frog can be killed 

 with ease by tying him out in the hot sun, for the cutaneous sur- 

 face must be kept continually moist in order to have its functions 

 preserved. This is insured in very dry weather by its power to 

 absorb a quantity of water which is stored away for use in an 

 internal nonurinary reservoir, from which receptacle it is excreted 

 over the surface of the body. When one suddenly picks up a frog 

 during the long, dry months of summer, it often voids a quantity 

 of this clear water in a succession of jets. The large, round eye 

 of the bullfrog is peculiar in some respects, for, if we tickle its 

 corneal surface with some light object, as a straw, it will be no- 

 ticed that the thick upper lid, covered as it is by the common in- 

 teguments, has very little movement, while on the other hand, as 

 the animal rotates its eyeball inward and beneath this, there at 

 the same time passes up over the organ the thinner, somewhat 

 transparent, lower eyelid. This shield, entirely covering the ball, 

 as it does, reminds one of the structure seen in birds, and called 

 the nictitating membrane. As soon as the irritation is with- 

 drawn, the animal again opens his eye, which, by the way, with 

 its truly beautiful iris, is, in my opinion, one of the most elegant 

 structures seen in Nature. Posterior to and below the eye we 

 meet with a flat, oval area, also covered by the skin, which is the 

 tympanum of the ear. One might possibly mistake this for a thin 

 flat bone in the skin, but this latter tissue in frogs is perfectly 

 smooth and is completely devoid of either scales or osseous plates. 

 There is an American genus of frogs, however {Ceratophrys), a 

 few representatives of which form an exception to this rule. If 

 we puncture the eardrum in the frog, it will be found that a fine 

 pig's bristle may be passed by a natural passage through the 

 opening made into the mouth. It goes through the Eustachian 

 tube, a canal which is also present in man and other vertebrates, 

 permitting, as it does, the vibrations of the tympanum. The fact 



