M'DoNNELL — On the Nervous System. 
47 
take up undulations or vibrations, and convert them into waves capable 
of being propagated along nervous tissue (neuricity, as it has been 
named). Thus, the same nerve tubule may be able to transmit along 
it vibrations differing in character, and hence, giving rise to different 
sensations ; and, consequently, the same nerve tubule may, in its normal 
condition, transmit the wave which produces the idea of simple contact, 
or that which produces the idea of heat — or, again, the same nerve 
tubules in the optic nerve which propagate the undulations of red may 
also propagate, in normal vision, those which excite the idea of yellow 
or blue, and so for other senses. 
I advocate this undulatory theory of sensation in preference to the 
theory of distinct conductors — 
Istly. Because it is simple. 
2ndly. Because it is strongly supported by analogy, when compared 
with wave propagations in other departments of science. 
3rdly. Because it appears to be in harmony with a large number 
of recognized physiological facts, which seem inexplicable 
upon the theory of distinct conductors. 
It would be obviously impossible, within the limits of one com- 
munication, to discuss such a theory in its application to the various 
senses. I wish merely to bring before the Academy, at present, a 
general statement of the grounds upon which this hypothesis rests ; and 
I shall hope, hereafter, in several communications, to elucidate its 
applicability to the transmission of the sensations peculiar to each special 
sense. 
1st. When compared with the theory of distinct conductors, the 
undulatory theory is obviously simpler as regards anatomical detail. 
Anatomy has not given any evidence that with an ordinary compound 
nerve there exist different kinds of conductors — to the highest powers of 
the microscope all such tubules are identical in appearance. 'Naj more, 
we now know that nerves may be so spliced (if I may use the expres- 
sion) on to one another, that sensitive nerves may be made continuous 
with those which convey the commands of the will to muscles. 
As regards the analogy between this theory of nerve action and 
the wave theory of light, I do not pretend to say that it holds in every 
respect : there are obvious points of difference. If we infer that light 
and heat do not consist of particles emitted by a hot body, our natural 
alternative is to suppose that they are undulations of a medium per- 
vading space. This hypothesis furnishes by far the best explanation 
of many very curious phenomena in light and heat, and is now generally 
received. This medium , which we suppose to pervade space likewise, 
with more or less freedom, pervades transparent and diathermanous 
bodies ; but nerve tissue being neither transparent nor diathermanous, 
it is not to be conceived that the undulations of this medium are trans- 
mitted along nerve tissue, as if through glass or rock salt : on the con- 
trary, the vibrations of light and heat are transferred from the medium 
in question to the axis cylinder of the nerve tubule in a form capable 
of being propagated to the sensorium. 
