THE GREAT AFFERENT SYSTEMS 311 



PROPRIOCEPTIVE PATHWAYS 



We have traced the course of the afferent impulses from the skin and from 

 the eye and ear to the cerebral cortex, and have learned that they play an es- 

 pecially important part in conscious experience. The stimulation of these ex- 

 teroceptive sense organs initiates both conscious and reflex adjustments of the 

 body to its environment. But the resulting movements serve to excite the 

 sensory nerve ending in the muscles, joints, and tendons; and any quick move- 

 ment or change in position of the head will also excite the nerve terminals in 

 the semicircular canals of the ear. From these sources afferent impulses pour 

 back into the nervous system along special paths to centers which to a great 

 extent are separate from those devoted to the exteroceptive functions and serve 

 to regulate the movements already initiated. The necessity for such regulation 

 is well illustrated by the ataxic gait of a tabetic in whom the afferent impulses 

 from the muscles, joints, and tendons are more or less completely lost. In a 

 sense the proprioceptive functions of the nervous system are secondary to the 

 exteroceptive, since the purpose of both is the proper adjustment of the organism 

 to its environment by means of reactions, called forth by external stimuli, 

 but regulated and controlled through afferent impulses arising within the 

 body. 



Since in the regulation of movement the proprioceptive subdivision of the 

 nervous system has to deal with constant factors, inherent in the arrangement 

 of the muscles, the resultant responses are more stereotyped and invariable in 

 character and are, for the most part, subconsciously executed. These reactions 

 belong more to the province of the cerebellum than to that of the cerebrum. 



Of the long ascending channels mediating afferent impulses from the muscles, 

 joints, and tendons, only one extends to the cerebral cortex by way of the thala- 

 mus; all the others end in the cerebellum. In fact, the cerebellum is the great 

 correlation center for afferent impulses of the propriceptive group, whether they 

 are conveyed by the vestibular nerve or the muscular branches of the spinal 

 nerves. 



It will be understood that on the motor side these two subdivisions of the 

 nervous system are not as distinct as on the afferent side. On the contrary, 

 both tend to discharge into common efferent systems. This is particularly true 

 of the primary somatic motor neuron, which serves as "the final common path" 

 for both. 



The Spinal Proprioceptive Path to the Cerebral Cortex. The conduction 

 system, along which those afferent impulses travel which underlie the rather 



