FUNCTIONS OF THE SENSORY GANGLIA MUSCULAR SENSE. 607 



effort he is making, can manage to stand and walk by looking at his limbs; 

 and the woman who cannot feel the pressure of her child upon her arms, 

 can yet sustain it so long as she keeps her eyes fixed upon it, but no longer, 

 the muscles ceasing to contract, and the limb drooping powerless, the 

 moment that the eyes are withdrawn from it. Thus it is, too, that when we 

 are about to make a muscular effort, the amount of force which we put forth 

 is governed by the mental conception of that which will be required, as in- 

 dicated by the experience of former sensations ; just as the contractions of 

 the muscles of vocalization are regulated by the conception of the sound to 

 be produced. Hence, if the weight be unknown to us, and it prove either 

 much heavier or much lighter than was expected, we find that we have put 

 forth too little or too great a muscular effort. 



537. There are two groups of muscular actions, however, which, although 

 no less voluntary in their character than the foregoing, are yet habitually 

 guided by other sensations than those derived from the muscles themselves. 

 These are, the movements of the Eyeball, and those of the Vocal apparatus. 

 The former are directed by the visual sense, 1 by which the action of the 

 muscles is guided and controlled, in the same manner as that of other mus- 

 cles is directed by their own " muscular sense ;" and hence it happens that, 

 when we close our eyes, we cannot move them in any required direction 

 without an effort that strongly calls forth the muscular sense, by which the 

 action is then guided. In persons who have become blind after having once 

 enjoyed sight, an association is formed by habit between the muscular sense 

 and the contractile action, that enables the former to serve as the guide after 

 the loss of the visual sense; but in those who are born perfectly blind, or who 

 have become so in early infancy, this association is never formed, and the 

 eyes of such persons exhibit a continual indeterminate movement, and can- 

 not by any amount of effort be steadily fixed in one spot, or be turned in 

 any definite direction. A very small amount of the visual sense, however, 

 such as serves merely to indicate the direction of light, is sufficient for the 

 government of the movements of the eyeball. In the production of vocal 

 sounds, again, that nice adjustment of the muscles of the Larynx, which is 

 requisite to the giving forth of determinate tones, is ordinarily directed by 

 the auditory sense: being learned in the first instance under the guidance 

 of the sounds actually produced ; but being subsequently effected volun- 

 tarily, in accordance with the mental conception (a sort of inward sensa- 

 tion) of the tone to be uttered, which conception cannot be formed, unless 

 the sense of hearing has previously brought similar tones to the minoL 

 Hence it is that persons who are born deaf are also dumb. They may have 

 no malformation of the organs of speech ; but they are incapable of utter- 

 ing distinct vocal sounds or musical tones, because they have not the guid- 

 ing conception, or recalled sensation of the nature of these. By long train- 

 ing, however, and by imitative efforts directed by muscular sensations in the 

 larynx itself, some persons thus circumstanced have acquired the power of 

 speech ; but the want of a sufficiently definite control over the vocal mus- 

 cles is always very evident in their use of the organ. It is very rarely that 

 a person who has once enjoyed the sense of hearing, afterwards becomes so 

 completely deaf as to lose all auditory control over his vocal organs. An 

 example of this kind, however, has been communicated to the public by a 

 well-known author, as having occurred in himself; and the record of his 

 experiences 2 contains many points of much interest. The deafness was the 



1 See Dr. Alison's Memoir on the Anatomical and Physiological Inferences from 

 the Study of the Nerves of the Orbit, in Trans, of Roy. Soc of Edinb., vol. xv. 



2 See the Lost Senses, by Dr. Kitto, vol. i, chapters 2 and 3. 



