334 FUNCTIONS OF THE NERVOUS SYSTEM. 



muscle, but upon the voluntary direction of the Sight towards the object to be 

 supported. Again, in the production of vocal sounds, the nice adjustment of 

 the muscles of the larynx, which is requisite to produce determinate tones, 

 can only be learned in the first instance under the guidance of the sensation 

 of the sounds produced, and can only be effected by an act of the will, in 

 obedience to a mental conception (a sort of inward sensation) of the tone to 

 be uttered, which conception cannot be formed, unless the sense of hearing 

 has previously brought similar tones to the mind. 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 uttering distinct vocal sounds or 

 musical tones, because they have not the guiding conception, or recalled sen- 

 sation, of the nature of these. By long training, and by efforts directed by the 

 muscular sense of the larynx itself, some persons thus circumstanced have 

 acquired the power of speech ; but the want of a sufficiently definite control 

 over the vocal muscles, is always very evident in their use of the organ. 



434. The conjoint movements of the two eyes, which concur to direct 

 their axes towards the same object, are among the most interesting of these 

 actions, in which Volition and Consensual action are alike concerned ; and 

 they afford an excellent illustration of the necessity for guiding sensations, 

 to determine the actions of muscles. The sensations, however, are not so 

 much those of the muscles themselves, as those received through the visual 

 organ; but the former appear capable of continuing to guide the harmonious 

 movements of the eyeballs, when the sense of sight has been lost. It 'is a 

 striking peculiarity of these movements, that, in the majority of them, two 

 muscles or combinations of muscles of opposite action are in operation at 

 once ; thus, when the eyes are made to rotate in a horizontal plane, the in- 

 ternal rectus of one side acts with the external rectus of the other. In most 

 other cases, there is a difficulty in performing two opposite movements, on 

 the two sides at the same time. Thus, if we move the right hand as if wind- 

 ing on a reel, and afterwards make the left hand revolve in a contrary direc- 

 tion, no difficulty is experienced; but if we attempt to move the two at the. 

 same time in contrary directions, we shall find it almost impossible. As the 

 Consensual movements of the Eyes are of sufficient interest and importance, 

 to require a detailed consideration, they will be examined more fully at the 

 close of the present section ( 450 456). 



435. If the preceding views be correct, we may regard the series of Gan- 

 glionic centres which have been enumerated ( 422, 423), as constituting the 

 real Sensorium ; each ganglion having the power of cummunicating to the 

 mind the impressions derived from the organ, with which it is connected, and 

 of exciting automatic muscular movements in respondence to these sensations. 

 If this position be denied, we must either refuse the attribute of consciousness 

 to those animals, which possess no other encephalic centres than these ; or 

 we must believe that the addition of the Cerebral hemispheres, in the Verte- 

 brated series, alters the endowments of the Sensory ganglia, an idea which 

 is contrary to all analogy. So far as the results of experiments can be relied 

 on, they afford a corroboration of these views. The degree in which animals 

 high in the scale of organization can perform the functions of life, without 

 any other centre of action than the Ganglia of Special sense, the Medulla 

 Oblongata, and the Cerebellum, appears extraordinary to those who are accus- 

 tomed to regard the Cerebral Hemispheres as the centre of all energy. From 

 the experiments of Flourens, Hertwig, Magcndie, and others, it appears that 

 not only Reptiles, but Birds and Mammalia, may survive for many weeks or 

 months (if their physical wants be duly supplied) after the removal of the 

 whole Cerebrum. It is difficult to substantiate the existence in them of 

 actual Sensation; but some of their movements appear to be of a higher kind 



