720 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



condition ; examples of it being furnished (as we have seen) by the closure of 

 the eyes to a dazzling light, the start caused by a loud and unexpected sound, 

 or the sneezing excited by sensory impressions on the Schneiderian membrane 

 or the retina. To these may be added the vomiting produced by various sensory 

 impressions, as the sight of a loathsome object, a disagreeable smell, a nauseous 

 taste, or that peculiar feeling of want of support which gives rise to " sea-sick- 

 ness," especially when combined with the sight of continually shifting lines and 

 surfaces, which itself in many individuals disposes to the same state ; the in- 

 voluntary laughter which is excited by tickling, and also that which sometimes 

 involuntarily bursts forth at the provocation of some sight or sound to which no 

 ludicrous idea or emotion can be attached ; the yawning which is excited by an 

 internal sensation of uneasiness (usually arising from deficient respiration), or by 

 the sight or sound of the act as performed by another ; and those involuntary 

 movements of the body and limbs, excited by uneasy sensations (probably 

 muscular), which are commonly designated as " the fidgets." When the reflex 

 activity of the Sensory ganglia is more strongly excited, in consequence either 

 of an unusual potency of the sensory impressions, or of an unusual excitability 

 of this part of the nervous centres, a much greater variety of sensori-motor 

 actions is witnessed. The powerful involuntary contraction of the orbicularis 

 and of the muscles which roll the eyeball upwards and inwards, in cases of 

 excessive irritability of the retina, is one of the best examples of this kind ; but 

 another very curious illustration is afforded by the involuntary abridgment 

 of the excito-motor actions of respiration when the performance of these is 

 attended with pain -the dependence of this abridgment upon the direct stimulus 

 of sensation, rather than upon voluntary restraint, being obvious from the fact 

 that it often presents itself on one side only, a limitation which the Will cannot 

 imitate. Again, there are certain Convulsive disorders which appear to depend 

 upon an undue excitability of these centres, the paroxysms being excited by 

 impressions which act through the organs of sense, and are not thus operative 

 unless the patient be conscious of them ; thus in Hydrophobia, we observe the 

 immediate influence of the sight, sound, or contact, of liquids, or of the slightest 

 currents of air, in exciting muscular contractions ; and in many Hysteric sub- 

 jects, the sight of a paroxysm in another individual is the most certain means of 

 its induction in themselves. A remarkable case of this general exaltation of 

 purely sensorial excitability has been recorded by Dr. Cowan, who gives the 

 following account of the phenomena, which can scarcely be referred to any other 

 than this category. " The shadow of a bird crossing the window, though the 

 blind and bed-curtains are closed, the displacement of the smallest portion of 

 the wick of a candle, the slightest changes in the firelight, induce a sudden 

 jerking of the spinal muscles, extending to the arms and legs when violent, and 

 this without the slightest mental emotion of any kind beyond a consciousness 

 of the movement. At times the vocal organs are implicated, and a slight cry, 

 quite involuntary, takes place. At these periods she is unusually susceptible of 

 all noises, especially the least expected and least familiar. Movements in the 

 next house inaudible to others, the slightest rattle in the lock of a door, tearing 

 a morsel of paper, and a thousand little sources of sound not to be catalogued, 

 induce results similar to those of visual impressions." 1 



749. It is, however, when the Cerebrum is not in a state which renders it 

 capable of receiving and acting upon Sensorial impressions, that we find the 

 independent reflex activity of the Sensory ganglia most strikingly displayed. 

 Thus in the Infant, for some time after its birth, it is obvious to an attentive 

 observer that a large part of its movements are directly prompted by sensations 

 to which it can as yet attach no distinct ideas, and that they do not proceed 



1 "Lancet," Oct. 4, 1845. 



