28 THE AMERICAN MONTHLY [February, 



The mere nerve fibre of the frog, sensitive as it is, is not sensitive 

 enough to be disturbed by so slight a difference as that between green 

 and bkie and red Hght, or to be excited Ijy the shadow cast by a fly, and 

 hence in so perfect a body as that of the frog, we find additional struc- 

 tures attached to the terminations of nerves going from the surface of 

 the body to the centre for the purpose of receiving extremely feeble 

 stimuli and impressing them upon the nerves. These are called end 

 organs. wSome end organs, e. g-.^ of touch, are not themselves peculiarly 

 sensitive, but only act as collectors or, as it were, focalizers of weak 

 stimuli, which, by them, are aggregated in sufficient strength to stim- 

 ulate the fibre ; others, e. £~., retinal cells, are, perhaps, peculiarly sensi- 

 tive in themselves, and after having been stimulated, stimulate the fibre 

 in connection with them. But the mode of action of the end cells is 

 not yet sufficiently understood. 



As a result of its specializations of irritability and the possession of 

 suitable organs and tissue, the frog is placed in very close relation with 

 his surroundings, andean care for his body in a wonderful degree. A 

 frog carefully decapitated so as to leave the cerebellum unharmed, can 

 jumj^ as well as a frog in perfect health, but he will not jump until he 

 is actually touched ; too late, perhaps, for self-preservation, but a frog 

 with all his powers will jump before he is touched, and, perhaps, long 

 enough before to escape capture. The headless frog can catch a fly if 

 it touches his nose, but the perfect frog catches a fly in the air several 

 inches distant. 



We thus look upon the nervous system as a vast number of specially 

 irritable cells peculiarly arranged, which co-operate to manage all the 

 parts of the body for the advantage of the whole. 



These tissues in the case of every individual fi'og are the product of 

 asexual reproduction from a few embryonic cells, the primitive nervous 

 cells, and they are thus like the protozoa in a mode of reproduction, as 

 well as in the power they have specialized. Powers of motion they 

 seem to have entirely abdicated, but metabolic power the\' have retained, 

 for they can construct from their food, the blood, the peculiar chemical 

 compounds of their own substance, and they can regulate and control 

 the oxidation of the tissue. 



We may then conclude that the nervous tissue, like the motor tissue, 

 is made up of cells which obey the general laws of cell life — which re- 

 semble all cells in sundry particulars, but wdiich have become v^ery 

 peculiar in form and function by reason of their high specialization of 

 the single function of irritability. Very much remains to be said about 

 the biology of nerve tissue, and very much more remains to be found 

 out. We have not space now to consider the conditions of irritability, 

 its heightening or lowering — the eflect of too frequent exercise of the 

 function, eftection. eflect of drugs on it. and many other inquiries, for 

 we propose onl}- to point out here that the frog is a cellular aggregate. 



Resjime. — W^e have now examined the frog's skin, his motor tissue, 

 and his nerve tissues, we find them all to be cellular aggregates, all 

 with certain common properties, and other special or peculiar oties, 

 which, however, arc not peculiar in kind but only in degree. We could 

 in the same way examine glands, bones, and other parts, and in every 

 case we should learn that protoplasmic cells -are the active units which 

 do the work of these organs, and whose exertions have produced the 

 organs themselves. 



