196 THE BIOLOGY OF THE FROG chap. 



the central nervous system provided the sympathetic nerves 

 which accompany the blood vessels remain uninjured. 



The condition of the pigment cells is profoundly influenced 

 by changes in the circulation. An arrest of the blood flow 

 causes a paling of the skin. If the leg of a dark-colored 

 frog be tightly ligatured around the knee, the part below the 

 ligature will soon assume a much lighter color. The same 

 result follows if the blood vessels alone are tied, and is 

 effected more quickly if the ligature is made around an 

 artery instead of a vein. 



Raising the temperature causes the pigment of the skin 

 to contract. Cold, on the other hand, causes the pigment 

 to expand and the skin to assume a dark color. The dark 

 color of winter frogs is in part at least the effect of cold, 

 and the lighter color of summer frogs in part the result of a 

 higher temperature. A dark-colored frog may readily be 

 made to turn much lighter if it is placed for several minutes 

 in water of a temperature 27 C. Changes of temperature 

 affect the concentration of pigment even in isolated pieces 

 of skin. 



Various chemical substances affect the chromatophores, 

 some causing a contraction, others an expansion of the pig- 

 ment. Carbon dioxide produces a darkening of the skin ; 

 carbon monoxide, on the other hand, causes the skin to turn 

 pale. Chloroform and some other anaesthetics as well as 

 certain irritants, such as croton oil and cantharides, cause 

 an expansion of the pigment on the parts of the skin to 

 which they are applied. Dryness tends to cause the skin 

 to turn pale, while immersion in water produces the reverse 

 effect. This has been observed especially in Rana fusca 

 by Biedermann and in R. agilis and Hyla by Werner. 



Biedermann has shown that color changes are influenced 

 in a remarkable way by contact stimuli. Specimens of 



