1881.] Comparative Neurology. 17 
susceptible to certain external influences, such tissue would 
quickly differentiate a portion as the path of least resistance, 
which would also be the most direct conductor of motions from 
without to the contractile part. Thus the neuro-muscular cells of 
Hydra appear. From the general mass proceeds the ectoderm, 
and from it is differentiated the nerve-muscle tissue. 
The causes of this differentiation may be conceived by regard- 
ing the forms assumed by a layer of the sporules of Lycopodium 
and sand, when this mixture is subjected to vibrations coarse 
enough to affect the layer. The electro-dynamic law which draws 
together matter transmitting currents tn one direction would of 
itself construct a nerve path to contractile tissue. 
A nerve, then, is internuncial only, and the ganglion cell is 
histogenetic. Neither have any force-producing power, but are 
both the media through which certain molecular vibrations are 
most swiftly transmitted. 
The primitive sense ts tactile and all senses have proceeded from 
ws differentiation For illustrative purposes let us consider energy 
as divided into molecular vibrations, from one ethereal pulsation 
in an eternity, to an infinite number of vibrations in one second. 
In such an undulatory series we may see, as a small division of 
it, all forces from sound to gravitation represented. While the 
protozoon may be visibly affected by every such undulation, the 
homogeneity of its composition prevents any differential re- 
sponse; for instance, the tremor of a musical note, heat, light, 
electricity, alike produce contractions or expansions (motions) of 
its mass. In a higher form of life nerve tissue appears, which 
conveys only certain vibrations and rejects all others. Take one 
undulation in a second as the capacity of this nerve fiber. Itisa _ 
tactile nerve. When a nerve fiber conveys more rapid undula- 
_ tions differentiation begins. Sixteen to forty thousand per second 
begin and end the auditory vibrations. Quicker vibrations to 
four hundred and fifty billion per second we may view as heat 
appreciation, thence to eight hundred billion from red to violet 
light, above this fluorescent undulations, “ chemical energy,” 
electricity, to infinity. We may thus mathematically conceive an 
1In a paper read before the American Association for the Advancement of Science, 
Boston, August 28th, 1880, published in full in the Journal of Nervous and Mental 
Disease for October, 1880, I treated this subject more with reference to the micro- 
Scopic anatomy of human nerve systems. Extracts, as above, made from that paper 
are such portions as refer more directly to our present subject. 
