592 THE HUMAN BODY. 



Whether, in addition to the true muscle sense, dependent 

 on afferent impulses sent to the brain from the contracted 

 muscle or its tendons, we have a more direct consciousness of 

 the amount of will exerted to produce a given muscular con- 

 traction, and can form thereby a judgment as to the extent 

 of the movement or effort, is a question still in dispute. A 

 main argument in favor of the existence of such centrally origi- 

 nating ' ' imiervatioii sensations ' ' is based on phenomena ob- 

 served in persons afflicted with paresis. They frequently 

 judge erroneously for a time as to the extent of movements made 

 by them, thinking that the movement is greater than it really 

 is. It is argued that in such cases the error cannot be based 

 on peripheral sensations, but must be due to the fact that 

 the person judges by the amount of volitional effort he has 

 made, which was such as in his previous condition of health 

 would have produced a greater muscular contraction than it 

 now does in his paretic condition. It is especially in connec- 

 tion with eye muscles that such errors have been noticed. 

 When we follow a moving object with the eyes we judge of 

 the rate of movement by the degree of contraction of the ocular 

 muscles needed to keep its image on the two f oveas : if the eye 

 muscles become suddenly enfeebled the person at first thinks 

 he turns his eyeballs faster than he really does and hence that 

 the object is moving faster than it actually does: or he may not 

 move his eye at all when he has willed to do so, and hence 

 conclude that stationary objects are in motion because their 

 images are still formed on the same region of the retina, which 

 could not be the case with stationary objects if the position of 

 the eyes were changed. 



Whether the sensations by which we judge the extent of a 

 muscular movement be entirely peripheral or in part central, 

 they enable us to determine very minute differences of con- 

 traction: the ocular determination of the distance of an object 

 not too far off to have its absolute distance determined with 

 considerable accuracy, depends almost entirely upon judg- 

 ments based upon very small changes in the degree of con- 

 traction of the internal and external straight (recti) muscles, 

 converging or diverging the eyeballs; and of the ciliary muscle 

 determining the necessary accommodation of the lens. A 

 singer, too, must be able to judge with great minuteness the 

 degree of contraction of the small muscles of the larynx nec- 

 essary to produce a certain tension of the vocal cords. It may 



