of the structure of fishes; frogs' eggs, tadpoles, and frogs have been almost universally used, while birds and mammals have been largely studied through our investigation of the fowl, the pigeon, and the rabbit. Of course, hundreds upon hundreds of other plant and animal organizations have been most exhaustively worked out, but students always, or very frequently, date back to the manuals upon the biology of these old standbys. Gray's Human Anatomy is a very good example of one of these aids to the study of a single vertebrate species—a monograph, as it were. Some day, Mivart's Cat will hold a similar place as a work of reference. I have already referred above to the use of the frog as an animal whereby the biological student may gain much information; indeed, I do not believe there is a single animal anywhere that, for this or other purposes, has come into more general use. If a common frog be fully studied in all its phases, and a comparative study be made of its entire structure, together with its physiology and habits, the student has made a long stride toward the comprehension of life processes in general, and will find himself landed far within the domain of biology, and very liberally equipped to investigate almost any problem zoölogy may have to offer. In addition to this, ever since physiology came to be a science, frogs have been used by the researchers in that department whereby to demonstrate some of the grandest truths that men have brought to light. Especially is this true with respect to the study of the muscular and nervous systems. Physicists also use them extensively, and the medical expert in experimenting with, or detecting the presence of, a variety of poisons. The circulation of the blood, and the processes of inflammation as seen under the microscope, are now studied by thousands of students in the laboratories the world over, and they are nowhere better seen than in the web of a frog's foot. Some of the more obscure actions of the heart have been made clear by the study of the entire circulatory apparatus of this useful batrachian. The action of woorara in destroying the properties of the motor nerves has been demonstrated by Bernard upon frogs; while Matteucci, by the use of a frog's leg, has shown the contrasted action of the direct and the inverse current. The remarkable experiments of Marshall Hall and of Pflüger, on the reflex action of the spinal cord, where the most delicate animal organization was necessary, were made possible by the use of frogs; while another experimenter has worked out through them the physiological action of strychnine. Indeed, some of the most important facts. in physiology have been, and constantly are to-day being, demonstrated upon frogs; and the list of such conquests, and the light they shed upon this most useful kind of knowledge, are altogether too long to enter upon here. Much is to be learned by simply studying the action
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POPULAR SCIENCE MONTHLY.