686 The Relations of Mind and Matter. [July, 
the connecting threads of protoplasm between the cells are doubt- 
less competent to convey motor impulses from cell to cell, and 
they very probably preceded the development of nerves as a sen- 
sory arrangement, permitting a slow and general transmission of 
motor influences to every part of the body. But if any particu- 
lar motions became habitual, through natural selection, the sen- 
sory inflow must have become to some extent specialized, follow- 
ing certain channels of conduction more frequently than others. 
But nutrition always attends activity, and in these special lines 
the fibrils must have grown larger and more capable. If their 
labor still increased, a second change must have succeeded. e 
line of special conduction being mainly composed of cells, with 
short interconnecting fibrils, a modification necessarily took place 
in the cells also. If the outer threads had continuous fibrillar 
connection through the cells, which we have some warrant to 
assume, these cell fibrilla must have grown larger and straighter 
as a result of extra nutrition and natural selection. They may, 
indeed, have exhausted the cell nutriment and caused the abor- 
tion of the remaining filaments. In short, a continuation of this 
process of evolution may have caused the gradual disappearance 
of most of the cells in the line of conduction, and the conversion 
of their fibrillæ into direct continuations of the developing nerve 
fiber. 
The make-up of the axis cylinder of every nerve fiber is in strong 
corroboration of this idea. It is found to consist of numerous 
extremely delicate fibrilla imbedded in a finely granulated sub- 
stance. Nuclei are also found in it. Thus it is closely analogous 
to the cell in composition, and presents strong indications of 
originating in a connected line of cells. Another feature of the 
nerve fiber is an interesting confirmation of this. The medullary 
sheath seems but a special elongation of the outer layer of the 
cells. It does not exist in the primitive nerve, of which we have 
probably a survival in the nerves of the sympathetic system, It 
seems the result of a fuller development, yet the fatty and albu- 
minous matters of which it consists are the substances which 
: exists most abundantly in the outer cell layer. Conversion into - 
matter is a general characteristic of deteriorating cells. Thus 
ie every Portion of the nerve fiber can be traced directly to the 
. cell, with singularly little change, and there is certainly much 
Teason to believe that nerve conduction is an outgrowth of a 
