8 



SENSATION. 



TERMINATION OF NKRVB-LOOPS IN 

 MUSCULAR FIBRES. 



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 fur- 

 nace, 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, under the guardianship 

 of the vetebrae, continually giving off branches of vari- 

 ous sizes, according to the work which they have to fulfill. 

 These branches ramify into smaller twigs, subdividing un- 

 til they become so small that they almost even baffle the 

 microscope. A familiar proof may be given of the won- 

 derfully 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 sensi- 

 bility of the finger tips is shown by the accom- 

 panying engraving, which exhibits the mode in 

 which the nerve-loops are distributed. The ob- 

 ject 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 number of ganglia, and from the constant disposition which it evinces 

 in its distribution to communicate and form small knots or ganglia." * It is wonder- 

 fully 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 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, 



* Wilson. 



NERVKS OP FINGER TIPS. 



