666 FUNCTIONS OP THE CEREBRO -SPINAL NERVOUS SYSTEM. 



movements can be at any time checked by an effort of the will, yet this does 

 not really indicate that the will has been previously engaged in sustaining 

 them ; since, for the will to act upon them at all, the attention must be re- 

 called to them, and the Cerebrum must be liberated from its previous self- 

 occupation. And the gradual conversion of a volitional into an automatic 

 train of movements, so that at last this train, once started, shall continue to 

 run down of itself, will be found to be less improbable than it would at first 

 appear, when it comes to be understood that the mechanism of both sets of 

 actions is essentially the same, and that they merely differ as regards the 

 nature of the stimulus which originally excites them ( 544). That the 

 same automatic movements are not excited by the same sensations, when 

 the Cerebrum is in its ordinary state of functional connection with the Seu- 

 sorium, is a fact entirely in harmony with the principle already laid down 

 ( 457, 458). The complete occupation of the mind in other ways, as in 

 close conversation or argument, or even (it may be) in the voluntary direc- 

 tion 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. 



536. 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 fur- 

 nishes an essential means of connection between the actions of the Cere- 

 brum 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 maybe affirmed with certainty 

 that no mental action can be originally excited, save by the stimulus of Sen- 

 sations ; and it is the office of the Sensory ganglia to form these out of the 

 impressions brought to them from the organs of sense, and to transmit such 

 sensorial changes to the Cerebrum. But they have a no less important par- 

 ticipation in the downward action of the Cerebrum upon the motor appa- 

 ratus ; for no voluntary action can be performed without the assistance of a 

 guidiny 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, and in which 

 the "muscular 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. [Sachs has demon- 

 strated that there are nerves of muscular sense, and that the perception of a 

 contraction is probably due to a mechanical irritation of sensitive nerves, 

 which run between and around the muscular fibres. 2 ] The proof of this 

 necessity is furnished by the entire impossibility of making or sustaining volun- 

 tary effort*, loithout a guiding sensation of some kind. Thus, in complete an- 

 aesthesia of the lower extremities, without loss of muscular power, the patient 

 is as completely unable to walk as if the motor nerves had also been para- 

 lyzed, 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 



1 See his chapter, On the Nervous Circle which connects the Voluntary Muscles 

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



2 [Recent Progress in Physiology, by Prof. II. P. Bowditch ] 



