124 LIFE: ITS NATURE AND ORIGIN 



membrane, however, acetylcholine will most likely have the same role 

 as in the active surface membrane of the fibers." 



The precise mechanism whereby the flow of current acts in the 

 effector organs is still uncertain. It might produce chemical 

 change locally, or transient differences in the charge or the orien- 

 tation of colloidal particles or macromolecules. As Volta long ago 

 showed, the contraction of a muscle follows its electrical stimula- 

 tion; but muscular relaxation follows cessation of the nerve 

 impulses: merely to stand upright requires the subconscious 

 innervation of many muscles. When unconsciousness supervenes 

 (as in narcosis or fainting), the nerve impulses cease, the muscles 

 relax, and the person falls limp. In the "rest cure" for tubercu- 

 losis, where it is desirable to keep exercise at the lowest possible 

 level, the patient reclines in a nearly prone condition. Actually, 

 the nerve impulses reach muscles at the rate of about 100 per 

 second, so that the contraction of the muscles involves great num- 

 bers of "twitches." When age or disease alters the tempo of the 

 twitches, tremor may become evident. Temporary nervous shock 

 often causes trembling. 



Professor George H. Parker of Harvard University has investi- 

 gated the activity of various chemical activators, originally termed 

 neurohumors by Henri Fredericq, which "really act as hormones 

 over shorter or longer ranges." In discussing the nervous control 

 of color in fishes, Parker goes back to Pouchet (1876), who showed 

 that if the integumentary nerves of a turbot are cut, the melanin 

 granules in its melanophores undergo a dispersion, spreading out 

 the color and darkening the skin. Parker further observes that 

 there are two classes of neurohumors: hydrohumors, soluble in 

 water and therefore found in blood, lymph, etc., and lipohumors, 

 soluble in lipoids, fats, oils, fat solvents, etc. He finds 63 that the 

 color changes of catfishes are controlled by three chief neuro- 

 humors: intermedin from the pituitary gland; acetylcholine; and 

 a concentrating neurohumor (probably adrenaline) from nerve 

 fibers controlling, respectively, concentration and dispersion. 

 Acetylcholine induces dispersion of melanophore pigment; adrena- 

 line causes the reverse. Both are lipoid-soluble, and acetylcholine, 

 in its fatty retreat, may thereby be protected from destruction by 

 cholinesterase; in fact, it accumulates to such an extent that its 

 effects may persist after nerve action has ceased, and it may be 

 extracted in measurable amounts from dark fish skins. "Thus the 

 fatty or lipoid substances in the animal body may serve as storage 



