242 On Cell Life. | April, 
from elements of a similar nature—from cells. Among the simplest 
organisms, philosophers have been in much doubt in which of the 
two kingdoms they shall place those beings consisting of but one or 
few cells, and perhaps there is nothing to justify the assertion of a 
sharp boundary. The distinction can only be clearly shown where 
one has to deal with complex organisms formed of many cells. 
The true characteristics of the two kingdoms are to be found in 
the manner in which the colony is built up by its individuals, and 
thus especially in that system of cells just mentioned which gives 
its peculiarity to the animal kingdom. This system is a series of 
cells widely spread through the whole body, in which the proto- 
plasmic matter is maintained in unbroken continuity throughout, 
by fine, long threads. It is the ‘nervous system.’ Let us consider 
this wonderful structure somewhat more closely, and indeed let us 
bring it before us in that form which it takes mm man, the highest 
of now known animals. The cells of the human nervous system 
are generally packed together in more or less numerous groups, 
which are called nerve-knots or ganglia, whence the name ganglion- 
cell is derived for the cells of the nervous system: they are simply, 
as all cells are, nothing more than protoplasmic lumps with nucleus 
and nucleolus. The nervous ganglia lie distributed over the whole 
body, in some places sparingly, in others profusely, and are bound 
together with connecting cords of the protoplasmic thread, which, 
as was said, always brings two cells into conjunction. Hach such 
protoplasmic thread is encased in a true sheath, and forms: with it 
what is called ‘a nerve-filament.’ It must further be noticed that 
each cell commonly is connected, not only with one, but often with 
many other cells by means of nerve-filaments. 
Besides these filaments, which establish the communication of 
the nerve-cells among one another, there are others which com- 
municate at one end only with a cell, and at the other end are 
connected with structures of a different nature. There are two 
sorts of these filaments, differmg very much in kind, though not in 
their essential nature. The one set—in the strictest meaning of 
the word—proceeds outwards from the organs of sense, and is 
destined to convey external impressions to the nervous system. 
The others, which terminate in the muscular fibres, are appro- 
priated to carrying the active influence of the nervous system to 
the exterior. 
We have now a complete picture of the nervous system in man, 
when we add that in him, as well as in the four higher classes of 
animals, by far the greatest number of the nervous elements—that 1s, 
of nerve-cells and connecting filaments—are packed together in a 
continuous mass, and shut up in the canal of the backbone and 
those cavities of the skull which are continuous with it. These 
great nervous masses are known as the brain and spinal-cord. 
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