SENSATION. <7 



be necessary to preserve their lives. And for all living beings the wandering of the mind but 

 for a few seconds would cause instantaneous death. 



All these difficulties are removed, and the animal kingdom preserved and vivified, by 

 means of certain vital organs, known by the name of nerves. 



It is clear enough that mind does not act directly upon the muscles and the various organs 

 of the material body, but requires a third and intermediate substance, by which it is enabled 

 to convey its mandates and to receive information. The necessarily multitudinous channels 

 through which this substance is conveyed are called "nerves," and are of a consistency more 

 delicate than that of any other portions of the animal 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 tierce 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. 



Towards the great nerve mass, called by the name of "brain," tend the nerve-cords that 

 supply the body with vital energy. It 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, imder the 

 guardianship of the vertebrae, continually giving off branches of various sizes, according to the 

 work which they have to ftilfil. These branches ramify into smaller twigs, subdividing until 

 they become so small that they almost even baffle the microscope. 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 accom- 

 panying 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 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 mimber of 

 ganglia, and from the constant disposition which it evinces in its distribution to communicate 

 and form small knots of 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 emo- 

 tional 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 con- 

 cussion that would hardly be felt upon 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, bring on collapse, deprive 



* Wilson. 



