308 ALFRED C. REDFIELD 



Why the anterior portion of these animals became pale is not 

 obvious. The effect cannot be due to a secretion of adrenin or 

 any other hormone, nor to the direct action of any environmental 

 factors, since these would affect all parts of the skin uniformly. 

 The coincidence of the point of operation upon the spinal cord 

 with the line of demarkation between the dark and pale areas of 

 the skin point to a causal connection between the two. The 

 conclusion is thus suggested that the posterior part of the body is 

 isolated by transection of the spinal cord from some nervous 

 disturbance which is acting upon the melanophores of the anterior 

 part of the body through nerves connecting directly with these 

 cells. These observations, although exceptional, lend strong 

 support to the conclusion drawn from the contraction of the 

 melanophore pigment of the leg by stimulation of the sciatic 

 nerve; the melanophores are acted upon directly by nerves, and 

 their pigment is contracted by nervous impulses. 



That the preceding interpretation is correct is shown clearly 

 by an experiment which demonstrates, in addition, the relation 

 existing between the adrenal glands, the nervous system and the 

 melanophores. It has been pointed out that cutting the nerves 

 distributed to a part of the skin does not interfere with the con- 

 traction of the melanophore pigment which follows noxious 

 stimulation. The contraction which occurs at this time has been 

 attributed to the action of the secretion of the adrenal glands 

 upon the melanophores. If the adrenal glands are removed 

 from horned toads which have had the nerves to a portion of the 

 skin transected, the isolated region is no longer affected by noxious 

 stimuli, although the melanophore pigment of the remainder of 

 the skin is made to contract. By this procedure the effect de- 

 scribed in the preceding experiment may be exactly duplicated 

 (fig. 16). 



August 9, 1916. Stimulated the mouth of a horned toad for five 

 minutes with a weak faradic current. A complete contraction of the 

 melanophore pigment resulted. 



Etherized the animal and transected spinal cord at thirteenth vertebra. 



August 11, 1916. Melanophore pigment was expanded. Stimu- 

 lated mouth as before for ten minutes. A uniform contraction of the 

 melanophore pigment resulted. The operation has not affected the 

 reaction of the pigment cells in any part of the skin. 



