DISEASES OF THE .NERVOUS SYSTEM. 113 



spinal cord, and in connection with the sympathetic system. Nerve 

 centers are classed as the automatic and the reflex centers, but these 

 two divisions are subdivided again according to the function of each 

 respective center of either of the great divisions. The action of an 

 automatic center occurs independent of any influence external to the 

 center itself. To illustrate the action of the reflex centers, the familiar 

 example of a piece of food accidentally getting into the larynx (or into 

 the windpipe, as it is popularly termed) may be considered. Nature has 

 endowed the mucous membrane the internal surface of the larynx with 

 the most exquisite sensitiveness, which is due to the terminal end organs 

 in the membrane being connected by means of their sensory nerves 

 with a nerve center. No sooner does the particle of food drop into tho 

 larynx than the terminal end organs receive the extraordinary irrita- 

 tion it causes, and the impression is conveyed by the fibers of the sen- 

 sory, or afferent nerve, to a nerve center in the brain, and from the cen- 

 ter the nervous impulse is sent by means of the fibers of the motor, or 

 efferent nerves, to the various muscles, the contraction of which causes 

 the forcible expulsion of air from the lungs, which dislodges and ejects 

 from the larynx the offending particle of food. For another example 

 the sensation of pain will suffice. If a finger comes in contact with tire 

 the sensation of paiu is received by the end organs of the sensory fibers 

 in the skin of the finger, and conveyed to the brain by the seuscry or 

 afferent fibers, and there is instantly carried by the motor or efferent 

 fibers to the muscles of the arm the impulse which causes the muscles 

 to snatch the finger from the fire. 



A nerve is a cord consisting of a certain number of fibers, inclosed in 

 a sheath of connective tissue. This sheath contains the blood-vessels 

 from which the nerve derives its nutrition. Large nerves are composed 

 of bundles of smaller ones, each of the smaller contained in its respective 

 sheath. Nerves divide and subdivide, sending off branches, which 

 ramify in all parts of the body, and, as they near their terminations, 

 they contain but one or two fibers. 



Nerves are the conductors of the nerve current, or impulse. 



The brain and ftpinal cord are contained within a bony canal, which 

 forms a protective covering for them. 



The spinal column consists of a number of bones, especially articulated 

 or joined one to the other, extending from the head to the tail. Through 

 each one of these bones the spinal canal is continued. 



The spinal cord, or spinal marrow, lodged within the spinal canal, is 

 continuous with the brain anteriorly, and terminates in a point in the 

 sacrum (that part of the spinal column which immediately precedes the 

 tail). The spinal cord is not of uniform size, it being considerably larger 

 at the part covered by the last two bones of the neek and the first two 

 bones of the back than it is immediately before or after this enlarge- 

 ment. It is again enlarged at the part covered by the bones in the 

 region of the loins. 



