THE NERVOUS SYSTEM 63 



Young (193 1, 1933) whose neurological work on teleosts 

 and elasmobranches points to the inadvisability of 

 drawing these distinctions in the autonomic system of 

 the lowest group of vertebrates with too great sharpness. 

 So far as these two classes of fibers are concerned, the 

 autonomic system in fishes is like a mother liquor out 

 of which has crystallized the much more definite auto- 

 nomic components — sympathetic and parasympathetic 

 in the higher vertebrates. 



The diversity of organization in chromatophoral 

 systems is apparent even more in the variety of neuro- 

 humors than in the types of innervations. As I have 

 already pointed out, it would be premature to attempt a 

 general consideration and classification of these activa- 

 tors. The most that can at present be done is to divide 

 them into the two groups of lipohumors and of hydro- 

 humors (Parker, 1935^). In Fundulus, Ameiurus, and 

 Mustelus the concentrating agents are lipohumors as 

 are the dispersing humors in Fundulus and in Ameiurus. 

 The ordinary hydrohumor from the pituitary gland dis- 

 perses melanophore pigment almost universally; that 

 from the medulla of the adrenal gland, adrenin, concen- 

 trates it with still greater uniformity. The parasympa- 

 thetic fibers to the vertebrate heart produce a hydro- 

 humor, acetylcholin, that inhibits the heart muscle, and 

 the same class of fibers in Fundulus produces a lipo- 

 humor that disperses melanophore pigment. When we 

 seek for generalizations in such an array of details we 

 find at present little beyond those associated with the 

 two types of solubilities already discussed. 



That agents like neurohumors are active in such gen- 

 eral central functions as the transmission of nerve im- 

 pulses from one neurone to another has for some time 

 been surmised. The production of a neurohumor on one 

 side of a synapse and its reception on the other may 



