The pigmentary effector system.—II

In a previous paper dealing with the pigmentary response evoked by pituitary (posterior lobe) administration in the common frog, emphasis has been laid on the necessity of discriminating between the alternatives of nervous and endocrine factors in controlling colour changes in Amphibia. In the present communication some account of the pigmentary responses evoked by physiological reagents is discussed, before passing on to more direct methods of experiment. To investigate the action of drugs on melanophores, the ideal method would be to study the effects of the latter on the isolated skin. In common with other investigators, we have, however, found that when frog’s skin is placed in Ringer’s solution a condition of extreme melanophore contraction supervenes, so that this mode of treatment is useless for demonstrating the action of drugs whose application induces expanded melanophores to contract. As Spaeth, who has studied the effect of drugs on the melanophores of fish scales, rightly insists, the administration of drugs to the intact animal is fraught with formidable objections. But the disadvantages are preeminently of the nature of limitations. Of these the most obvious are: first, the difficulty in discriminating between local, peripheral, central, or reflex effects, when the seat of action of the drug is not homogeneous,e .g . in the case of atropine which has peripheral (parasympathetic) as well as central effects, or of veratrine, which acts directly on contractile tissue, and reflexly through its excitatory effect on sensory nerve endings; secondly, the possibility of inducing death, or interfering unduly with the circulation, with quantities subminimal to produce pigmentary response. However, if these limitations are recognised, the study of drug administration can, in the light of existing pharmacological knowledge, be used to explore the possible existence of a nervous mechanism of control in colour change. In this connection both direct experiment and the study of drug administration, have prompted a bewildering series of conflicting statements from a body of investigators so numerous that it is impossible to mention them individually. Fuchs, in his extensive survey of the literature—there are about 150 papers on Amphibian pigmentary changes—comments upon this lack of unanimity: “Wir müssen diese verschiedene Reaktionen auf ein und dasselbe Reagens als physiologische Artverschiedenheiten ansehen.” Foremost among those who have studied pigmentary responses to drugs in Amphibia are Biedermann, Bimmermann, Carnot, and Fuchs (1906). The reagents investigated by previous workers include veratrine, strychnine, santonin, curare, brucine, coniine, morphine, atropine, and eserine; but none have systematically applied modern knowledge of pharmacological reagents, using those whose action is most instructive. Apart from Lieben’s important work (1906) on adrenalin, the only observation which is specially interesting is Fuchs’ discovery that nicotine in amount adequate to produce motor paralysis induces darkening of the frog’s skin.