MELANOPHORES OF THE HORNED TOAD 311 



Nervous impulses reach the part of the cord in question from 

 either end. In the case of noxious stimuH they no doubt travel 

 over sensory nerves. Impulses owing their origin to the color of 

 the environmental substratum probably travel over the optic 

 nerves and thence posteriorly through the brain and spinal cord. 



Nervous impulses pass out from this region either to the 

 adrenal glands or to efferent nerves which connect directly with 

 the melanophores. Since the experiments cited above indicate 

 that the portion of the spinal cord anterior to the eighth vertebra 

 cannot produce a reaction of the melanophores, and the portion 

 posterior to the thirteenth vertebra is unnecessary for a reaction, 

 those impulses which pass to the adrenal glands must traverse 

 nerves which leave the cord between the eighth and thirteenth 

 vertebrae. 



The course of those impulses which pass directly to the melano- 

 phores is indicated roughly by the experiment described on page 

 308. Since it is possible to isolate the posterior portion of the 

 skin, from these impulses by simply cutting the spinal cord, it 

 may be concluded that the nerve tracts which carry impulses to 

 the melanophores run posteriorly, and probably anteriorly, within 

 the spinal cord to the segments of the body in which the affected 

 melanophores lie. From these points they leave the cord to 

 traverse segmental peripheral nerve-trunks to the melanophores. 



What class of nerve fibers carry these impulses along the 

 peripheral nerves? The fact that the melanophores are coordi- 

 nated through the activity of adrenin indicates that the sympa- 

 thetic division of the automonic nervous system is involved. 

 After comparing the action of sympathetic nerves and that of 

 adrenin upon a long list of organs, Elliott ('05, p. 466) states: 

 ''A positive reaction to adrenalin is a trustworthy proof of the 

 existence and nature of sympathetic nerves in any organ." 

 Spaeth ('16a) has shown that there is a very close analogy between 

 the physiological behavior of melanophores and smooth muscle, a 

 tissue which is almost invariably innervated by the sympathetic 

 division of the autonomic nervous system. 



