8 SENSATION. 
frame. There is a rather striking and close analogy between the mode in which the 
three systems of mind, nerve, and muscle act together, and the working of a steam- 
engine. In the engine we may take the fire as the analogue of the mind ; the water, of 
the nervous substance—the water-tubes representing the nerves; and the iron and brass 
machine as the representative of the bone and muscle. Thus we may make as large 
a fire as we like, heap on coals, and urge a fierce draught of air through the furnace, 
until the grate is filled with a mass of glowing white-hot matter. But the fire cannot 
act on the wheels without the intermediate substance, the water. This medium being 
supplied, the fire acts on the water, and the water on the metallic bars and wheels, 
so that the three become one harmonious whole. 
The great nerve mass, called by the name of “ brain,” is 
the general source of all the nerve-cords that supply the 
body with vital energy, and seems to be the nerve-heart, so 
to speak. From the brain, a cord of nervous matter, called 
the “spinal cord,” runs along the back, under the guardian- 
ship of the vertebra, continually giving off branches of 
various sizes, according to the work which they have to fulfil. 
These branches ramify into smaller twigs, subdividing until 
EL pera Aa they become so small that they almost even baffle the micro- 
scope. A familiar proof may be given of the wonderfully minute 
subdivision of the nerves, by trying to probe the skin with the point of a fine needle, 
and to discover any spot so small that the needle-point does not meet with a nerve. 
The cause of the peculiarly delicate sensibility 
of the finger tips is shown by the accompanying 
engraving, which exhibits the mode in which the 
nerve-loops are distributed. The object is greatly 
magnified, the two ridges being the enlarged 
representations of the minute raised lines which 
appear on the tips of the fingers and thumbs. 
That the nerves all find their way to the brain 
\ and issue from thence, is plainly shown by the 
ee ey eEn nine. well-known fact that if the spinal cord be injured 
all sensation ceases in the parts of the body that 
lie below the injury. And it is possible to deprive any limb of sensation by dividing 
the chief nerve that supplies that member with nerve-fibres. 
There seem to be two sets of nerves for the two purposes of conveying motive 
power to the body and of bringing to the nervous centres the sensations of pain or 
pleasure felt by any part of the body. These are appropriately known as nerves of 
motion and nerves of sensation. 
Connected with these nerves is a second system of a very curious nature, known 
by the name of the “sympathetic nerve.” The greater portion of the sympathetic 
nerve in the human frame “communicates with the other nerves immediately at their 
exit from the cranium and vertebral canal. It is called the ganglionic nerve, from 
being constituted of a number of ganglia, and from the constant disposition which it 
evinces in its distribution to communicate and form small knots or ganglia.”* It is 
wonderfully interwoven with the vital organs, and from this disposition it is sometimes 
termed the “organic nerve.” Its functions are closely connected with the phenomena 
of organic life, and it seems to be especially sensitive to emotional disturbances. There 
are several aggregations of the ganglia in various portions of the body; the largest, 
which is known by the name of the “solar plexus,” is placed in the pit of the 
stomach *or “epigastrium.” Its importance may be easily inferred from the extreme 
agony that is caused by the slightest blow near the region of that group of ganglia. 
A concussion that would hardly be felt wpon any other portion of the body, will, if 
it takes place on the epigastrium, at once cause the injured person to fall as if shot, 
* Wilson, 
