G64 FUNCTIONS OF THE CEREBRO-SPINAL NERVOUS SYSTEM. 



the most part only when this upward transmission is checked, either by the 

 non-development or the functional inactivity of the Cerebrum, or by its com- 

 plete occupation in some other train of action, or, on the other hand, when the 

 reflex action of the Sensory ganglia is called into play with unusual potency, 

 that we have any manifestations of the sensori-motor or consensual mode of 

 operation in Man, that are at all comparable in variety or importance to 

 those instinctive acts which are so remarkable in the lower animals ( 448). 

 533. Still, sufficient evidence of the existence of this class of reflex move- 

 ments may be drawn from observation of the actions of Man in his ordinary 

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

 of the eyes to a dazzling light, the staiA caused by a loud and unexpected 

 sound, and the sneezing excited by sensory impressions on the Schueideriau 

 membrane or on the Retina. To these may be added the vomiting produced 

 by various sensory impressions, as a sight of a loathsome object, a disagreeable 

 smell, a nauseous taste, or that peculiar feeling of want of support which gives 

 rise to " sea -sickness," especially when combined with the sight of continually 

 shifting lines and surfaces, which itself in many individuals disposes to the 

 same state; the involuntary laughter which is excited by tickling, and also 

 that which sometimes 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 de- 

 ficient 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 seusori-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 

 ( 517), 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 de- 

 pend upon an undue excitability of these centres, the paroxysms being ex- 

 cited 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 subjects, 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 its 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 caudle, the slightest 

 change in the firelight, induce a sudden jerking of the spinal muscles, ex- 

 tending 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 



