632 Dynamic Theory. 



standing the great influence memory stimuli have in our actions, how- 

 ever, there are still a good many of these sensori-motor movements, ex- 

 amples of which are the eyes closing to a sudden dazzling light ; the 

 start caused by an unexpected sound ; sneezing, from excitation of the 

 retina by a dazzling light, and of the pituitary membrane by an irri- 

 tating substance, such as snuff ; vomiting, produced by a loathsome ob- 

 ject, a disagreeable smell, or a nauseous taste ; sea-sickness and nausea, 

 from swinging, teetering, and riding backwards ; laughter upon tick- 

 ling ; yawning from uneasiness, arising generally from deficient respira- 

 tion, but sometimes in the unconscious imitation of others doing the same 

 thing ; fidgets from an uneasy feeling in the muscles. In disease, es- 

 pecially nervous disease, there is much of this sensori-motor action. In 

 hydrophobia, the sight, sound or touch of liquids, or even currents of 

 air, are liable to excite the muscular contractions, and in many hysteric 

 subjects the sight of a paroxysm in another is pretty certain to bring 

 it on themselves. A hysteric case is cited in which a woman was liable 

 to be thrown into convulsive movements, sometimes accompanied by an 

 involuntary cry, by the slightest external stimulus of sight or sound, the 

 shadow on a curtain of a passing bird, flickering of a flame in the fire- 

 place, displacement of a portion of the wick of a candle, the rustle of 

 a paper or rattle of a door lock. ( Carpenter. ) 



' s lt may be affirmed with certainty, that no mental action can be orig- 

 inalty excited save by stimulus of the sensations, 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 participation in 

 the downward action of the cerebrum upon the motor apparatus, for no 

 voluntary action can be performed without the assistance of a guiding 

 sensation, as was first pointed out by Sir C. Bell." The full signifi- 

 cance of this is that our actions are so essentially and rigidly reflex that 

 no movement of a single muscle can be made by the guidance of the 

 will, or of the idea motor or cerebral stimuli. A direct and present 

 sensation or stimulus from the periphery is necessary in every moment 

 of muscular action. " In the majority of cases the guiding or controlling 

 sensation is derived fiom 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 is not 

 adequate alone to furnish the necessary stimulation. 



It is impossible to make or sustain voluntary efforts without a guiding 

 sensation of some kind. ( Carpenter. ) * < Thus, in complete anaesthesia 

 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- 



