MELANOPHORES OF THE HORNED TOAD 313 



The facts tend to show that in many cases postural contraction (as 

 Sherrington designates tonus) is astonishingly economically maintained; 

 that the turnover of chemical energy involved by it is extremely low. 

 In such cases its relative unfatiguability may well be related to its 

 economy of maintenance. So strikingly does this aspect of it contrast 

 with the expense of maintenance and the relatively rapid fatiguability 

 of ordinary tetanic contraction as to suggest that the chemico-physical 

 process underlying postural contraction is in part at least essentially 

 other than that underlying twitch contraction, and the fusion of twitch 

 contractions termed tetanic contraction. 



The supposition has been put forward that in maintaining this eco- 

 nomical postural contraction the muscle-fiber or some part of it, clots, 



changes from sol to gel On this view evidently one of 



the nerves . . .' . can cause the contents of its muscle-fibers to 

 gel, to solidify, another of its nerves cause it to unclot. 



Such a view is easily applicable to visceral muscle such as vesical and 

 gastric, and to that of the blood-vessel wall. 



It appears, therefore, that there is a marked similarity between 

 the contraction and expansion of melanophore pigment and the 

 tonic changes in the state of smooth muscle. That the con- 

 traction of the melanophore pigment resembles the prolonged 

 tonic contraction of smooth muscle more closely than the momen- 

 tary contraction of skeletal muscle is demonstrated by the fact 

 that the skin of the horned toad will remain pale for months if 

 the animals are kept in a light colored environment. There is no 

 evidence of any fatigue of the melanophore from prolonged con- 

 traction, or expansion, of its pigment. Spaeth ('16c) has made a 

 similar observation upon the melanophores of Fundulus. 



Spaeth has shown that the state of aggregation of melanin 

 granules within the pigment cell may be altered by a variety of 

 chemical and physical stimuli as well as by the physiological 

 action of nerve impulses. Antagonisms are demonstrated 

 between not only chemical substances, e.g., sodium and potas- 

 sium, but between chemical and physical stimuli, e.g., atropine 

 and heat (Spaeth, '16b). The tonic state of the melanophore, 

 that is the degree of expansion or contraction of its pigment, may 

 be conceived to depend upon the resultant efTect of the various 

 conditions which exist within and about the cell upon its state of 

 colloidal aggregation. Any change in these conditions will upset 

 the equilibrium which has existed ; a new point of equihbrium will 



