722 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



direction of some other train of muscular movements, is no less favorable than 

 the state of reverie to that independent action of the Automatic centres which 

 has been now described. 



750. In the state of entire functional activity of the nervous centres of Man, 

 however, there can be no doubt that the operation of the Sensory Ganglia is 

 entirely subordinated to that of the Cerebrum; and that it furnishes an essential 

 means of connection between the actions of the Cerebrum on the one hand, and 

 those of the organs of Sense and Motion on the other, by the combination of 

 which the Mind is brought into relation with the external world. For, in the 

 first place, it may be affirmed with certainty that no mental action can be excited 

 in the first instance, save by the stimulus of Sensations; and it is the office of 

 the Sensory ganglia to form these out of the impressions brought to them from 

 the organs of sepse, and to transmit such sensorial changes to the Cerebrum. But 

 they have a no less important participation in the downward action of the Cere- 

 brum upon the motor apparatus; for no voluntary action can be performed without 

 the assistance of a guiding Sensation, as was first prominently stated by Sir C. 

 Bell. 1 In the majority of cases, the guiding or controlling sensation is derived 

 from the muscles themselves, of whose condition we are rendered cognizant by 

 the sensory nerves with which they are furnished ; but there are certain cases in 

 which it is ordinarily derived from one of the special senses, arid in which the mus- 

 cular sense can only imperfectly supply the deficiency of such guidance; whilst 

 again, if the muscular sense be deficient, one of the special senses may supply 

 the requisite information. The proof of this necessity is furnished by the entire 

 impossibility of making or sustaining voluntary efforts without a guiding sensa- 

 tion of some kind. Thus, in complete anaesthesia of the lower extremities, with- 

 out loss of muscular power, the patient is as completely unable to walk as if 

 the motor nerves had also been paralyzed, unless the deficient sensorial guidance 

 be replaced by some other; and in similar affections of the upper extremities, 

 there is a like inability to raise the limb or to sustain a weight. But in such 

 cases, the deficiency of the "muscular sense" may be made good by the visual; 

 thus the patient who cannot feel either the contact of his foot with the ground, or 

 the muscular effort he is making, can manage to stand and walk if he look at his 

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

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

 the muscles ceasing to contract, and the limb dropping 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 indicated by the ex- 

 perience of former sensations ; just as the contractions of the muscles of vocaliza- 

 tion 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 mus- 

 cular effort. 



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

 voluntary in their character as 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, 3 by which the action of the muscles is guided and 

 controlled, in the same manner as that of other muscles is directed by their own 

 "muscular sense ;" and hence it happens that, when we close our eyes, we cannot 



1 See his chapter "On the Nervous Circle which connects the voluntary muscles with 

 the Brain," in his work " On the Nervous System of the Human Body." 



2 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. 



