FREE BOOKS

Author's List




PREV.   NEXT  
|<   30   31   32   33   34   35   36   37   38   39   40   41   42   43   44   45   46   47   48   49   50   51   52   53   54  
55   56   57   58   59   60   61   62   63   64   65   66   67   68   69   70   71   72   73   74   75   76   77   78   79   >>   >|  
g both the structure and the working of the nervous system to keep in mind that it contains _but one fundamental unit of structure_. This is the neurone. Just as the house is built up by adding brick upon brick, so brain, cord, nerves and organs of sense are formed by the union of numberless neurones. [Illustration: FIG. 6.--Neurones in different stages of development, from _a_ to _e_. In _a_, the elementary cell body alone is present; in _c_, a dendrite is shown projecting upward and an axon downward.--After DONALDSON.] THE NEURONE.--What, then, is a neurone? What is its structure, its function, how does it act? A neurone is _a protoplasmic cell, with its outgrowing fibers_. The cell part of the neurone is of a variety of shapes, triangular, pyramidal, cylindrical, and irregular. The cells vary in size from 1/250 to 1/3500 of an inch in diameter. In general the function of the cell is thought to be to generate the nervous energy responsible for our consciousness--sensation, memory, reasoning, feeling and all the rest, and for our movements. The cell also provides for the nutrition of the fibers. [Illustration: FIG. 7.--Longitudinal (a) and Transverse (b) section of nerve fiber. The heavy border represents the medullary, or enveloping sheath, which becomes thicker in the larger fibers.--After DONALDSON.] NEURONE FIBERS.--The neurone fibers are of two kinds, _dendrites_ and _axons_. The dendrites are comparatively large in diameter, branch freely, like the branches of a tree, and extend but a relatively short distance from the parent cell. Axons are slender, and branch but little, and then approximately at right angles. They reach a much greater distance from the cell body than the dendrites. Neurones vary greatly in length. Some of those found in the spinal cord and brain are not more than 1/12 of an inch long, while others which reach from the extremities to the cord, measure several feet. Both dendrites and axons are of diameter so small as to be invisible except under the microscope. NEUROGLIA.--Out of this simple structural element, the neurone, the entire nervous system is built. True, the neurones are held in place, and perhaps insulated, by a kind of soft cement called _neuroglia_. But this seems to possess no strictly nervous function. The number of the microscopic neurones required to make up the mass of the brain, cord and peripheral nervous system is far beyond our mental grasp. It is computed that
PREV.   NEXT  
|<   30   31   32   33   34   35   36   37   38   39   40   41   42   43   44   45   46   47   48   49   50   51   52   53   54  
55   56   57   58   59   60   61   62   63   64   65   66   67   68   69   70   71   72   73   74   75   76   77   78   79   >>   >|  



Top keywords:

neurone

 

nervous

 
dendrites
 
fibers
 

diameter

 
function
 

neurones

 
structure
 

system

 

DONALDSON


NEURONE
 

Illustration

 

distance

 
Neurones
 
branch
 

freely

 
extend
 

spinal

 

FIBERS

 
greater

angles

 
approximately
 
slender
 

comparatively

 

length

 

greatly

 

parent

 

branches

 
NEUROGLIA
 

possess


strictly

 

neuroglia

 

cement

 

called

 
number
 

microscopic

 

mental

 
computed
 

required

 
peripheral

insulated

 

invisible

 

extremities

 

measure

 
microscope
 
entire
 

element

 
structural
 
larger
 
simple