60 COLOR CHANGES IN ANIMALS 



fied smooth-muscle cells. The sparse and scattered 

 innervation of smooth-muscle cells with only a nerve- 

 terminal here and there in a wealth of cells, is in strong 

 contrast with the multitude of nervous end-organs that 

 surround even a single chromatophore (Fig. 27). 



The relation of chromatophores to nerves is extremely 

 diverse. In amphibians these color-cells are probably 

 without direct nervous control and are adjusted entirely 

 through pituitary neurohumors. In the dogfish, Mus- 

 telus, blanching is a nervous function, and darkening 

 results from a pituitary secretion. In the killifish, Fun- 

 dulus, both blanching and darkening are nervous. If 

 we accept the melanophore system of the killifish with 

 its concentrating and dispersing nerve-fibers as the more 

 usual type, we must turn to other kinds of muscle than 

 smooth muscle for comparison. The best of these is 

 the vertebrate heart-muscle. This muscle, like smooth 

 muscle, is under the control of the autonomic nervous 

 system. Moreover it has a double innervation, sym- 

 pathetic and parasympathetic. The sympathetic fibers 

 of the heart accelerate its action, the parasympathetic 

 inhibit it. From this standpoint the concentrating 

 fibers of melanophores are believed to come under the 

 same category as the sympathetic fibers of the heart, 

 and the dispersing fibers under the same as the para- 

 sympathetic. Much can be said for this comparison, 

 but the heart as a muscle is not the typical tonus organ 

 that the melanophore is. Further the parasympathetic 

 or inhibitory fibers of the heart appear to act on that 

 muscle through acetylcholirt, a substance which in all 

 respects fulfills the requirements of a neurohumor and 

 yet appears to have only a very slight effect upon me- 

 lanophores (Parker, 1934c), an effect which as a matter 

 of fact is the reverse of what was to have been expected. 

 Thus the comparison between chromatophores and heart 



