SENSATIONS AND THE SENSE ORGANS 137 



by the optic pathway give rise to visual sensation, while 

 those from the upper part of the nose are responsible 

 for olfactory impressions. One would be inclined to 

 say that the impulses must be of the most widely varied 

 character. It is, therefore, a surprise to the student 

 to hear that most physiologists have adhered to the 

 view that they are essentially uniform in all nerves. 



How can we account for the facts before us without 

 admitting that the impulses can differ in kind? It 

 should be said frankly that this may be impossible, but 

 we always prefer a simpler conception to one which is 

 more involved unless we are compelled to adopt the 

 latter. If nerve-impulses are all alike, our emphasis 

 must be upon the places to which they go rather than 

 upon the currents themselves. We shall find that there 

 are analogies to help us in the attempt to do this. 



The lighting of a gas jet and the ringing of a bell are, 

 perhaps, as different as the movement of a muscle and 

 the act of secretion performed by a gland. Both may 

 be caused by electric currents produced in similar 

 batteries but led through different and suitable fix- 

 tures. No one would argue that the current used to 

 light the gas had any specific gas-lighting virtue in 

 itself; it could as well do something else. This is parallel 

 with what is known as the Mullerian doctrine in re- 

 gard to the nervous system: that nerve-impulses vary 

 only in intensity not in kind and that they produce 

 one effect or another according to the particular struc- 

 tures on which they finally act. 



The impulses in the fibers of the optic nerve have been 

 assumed to be of the same order as those in the auditory. 

 Upon this assumption it has been suggested that if 

 impulses having their origin in the retina under the 

 influence of light could be switched over into the path 

 from the organ of hearing and so arrive at the hearing 

 centers of the cerebrum anything ordinarily visible 

 would be audible; one could listen to lights and colors. 

 There is another consequence of the Mullerian principle 



