230 THE NERVOUS SYSTEM 



medullary substance and divide to form arborizations which are 

 flattened into the form of a leaf. 



(e) The corpuscles of Golgi are situated at the point of union 

 of tendons with muscles, and are believed by some to have to do 

 with the muscular sense. They are flattened fusiform bodies 

 composed of granular substance enclosed in layers of hyaline 

 membrane and containing nervous fibrillae. 



Properties and Classification of Nerve Fibers. Nerve 

 fibers are for the purpose of conveying messages either peripher- 

 ally or centrally. They may be stimulated to action by anything 

 capable of suddenly increasing their irritability. In any case 

 the effect of the stimulus, whether normal or abnormal, is mani- 

 fested at the peripheral distribution of the stimulated fiber. So 

 far as most external manifestations are concerned, nerves may 

 be classified as motor and sensory. That is to say, stimulation, 

 for instance, of a cerebro-spinal nerve (except those of special 

 sense) is followed, under ordinary conditions, by one of two 

 results there is either pain or contraction of a muscle to which 

 the nerve is distributed. This is a typical illustration of the 

 action of motor and sensory fibers, and the manifestation of 

 nerve action, whether it consists in pain or motion, is a result 

 only of the conduction of an impression of an impulse to the 

 center or the periphery. It is to be noted that the result of thus 

 stimulating a nerve fiber is manifested at one extremity only of 

 that fiber, and always at the same extremity. 



However, since there are nerve fibers the stimulation of which 

 is not followed by pain or motion, the division into sensory and 

 motor fibers is not comprehensive enough to include all the 

 fibers in the body. But since, as above stated, the only office of 

 fibers is to conduct, and since they always conduct in a direction 

 either toward or away from the center, all nerves may be classified 

 as either centripetal or centrifugal. A corresponding division 

 is into afferent and efferent. It will be seen that all motor 

 fibers are centrifugal or efferent, but not all centrifugal or efferent 



