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Page:Popular Science Monthly Volume 49.djvu/196

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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 important part of the respiratory apparatus, and a frog can be killed with ease by tying him out in the hot sun, for the cutaneous surface 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 noticed that the thick upper lid, covered as it is by the common integuments, 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 As soon as the irritation is withdrawn, the animal again opens his eye, which, by the way, with its truly beautiful iris, is, in my opinion, one of the most elegant structures seen in Nature.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