THE NERVOUS SYSTEM 61 



muscle is as inadequate as that between these cells and 

 smooth muscle. The truth is that chromatophores, 

 though they have some similarities with smooth muscle 

 and others with heart muscle, differ from both to such 

 an extent that they must be regarded as a type of tissue 

 sui generis. They are in no sense to be classed with 

 any kind of muscle. 



Is the double autonomic innervation of chromato- 

 phores, such as is seen in Fundulus and in Ameiurus, 

 to be regarded as sympathetic and parasympathetic? 

 This question is not easily answered. It has been stud- 

 ied by Smith (1931a) from the standpoint of the action 

 of drugs. Smith found that cocaine, a stimulus for 

 sympathetic fibers, induced a concentration of pigment 

 in the melanophores of Fundulus, and that ergot, a sym- 

 pathetic depressant, checked this concentration, results 

 which thus favored the view that concentrating fibers 

 belong to the sympathetic division of the autonomic 

 system. In a corresponding way pilocarpin and physo- 

 stigmin, both parasympathetic stimulants, called forth 

 pigment dispersion and this was retarded by atropin, a 

 parasympathetic depressant. These observations, so 

 far as they go, point to an affirmative answer to the 

 question at the beginning of this paragraph, but experi- 

 ments with drugs are always precarious, and Smith in 

 his final declaration is cautious not to draw too definite 

 a conclusion. 



In the dogfish Mustelus only one set of nerve-fibers 

 is present, and they are concentrating fibers. In con- 

 formity with what has been said about Fundulus these 

 fibers should belong to the sympathetic division of the 

 autonomic system. Following the general concepts of 

 vertebrate neurology they would be classed as post- 

 ganglionic efferent fibers whose cell-bodies lie in appro- 

 priate autonomic ganglia and whose axons, as non- 



