708 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



if not impossible, to remove one or both of these ganglionic masses, without 

 doing such an injury to the Crura Cerebri on which they repose, as shall in 

 great degree account for such disturbed movements ( 738). Irritation of one 

 of the Tubercula Quadrigemina has been observed, both by Flourens and Longet, 

 to produce contraction of the pupils of both eyes. These results of experiment 

 are partly confirmed by Pathological phenomena in Man ; for there are many 

 instances on record in which blindness has been one of the consequences of 

 diseased alterations in one or both tubercles ; and in some of the cases in which 

 the lesion extended to parts seated beneath the tubercles, disturbed movements 

 were observed. The subservience of these bodies to the exercise of the visual 

 sense appears, on the whole, to be the point best established in regard to their 

 functions ; and considering the degree in which this sense is concerned in the 

 regulation of the general movements of the body, it is not surprising that 

 lesions of its centre should occasion a perversion of these movements. This 

 appears the more probable from the fact that, in animals whose Sensory ganglia 

 bear so large a proportion to the whole Encephalon, that we must look upon 

 them as the principal centres of motor activity, instead of being chiefly concerned 

 (as in Man) in the mere guidance of movements whose origin is Cerebral, 

 lesions of the organ of sense, from which the impressions that excite the sen- 

 sori-motor impulses are derived, produce a corresponding disturbance. Thus 

 Flourens found that a vertiginous movement may be induced in Pigeons by 

 simply blinding one eye ; and Longet produced the same effect by evacuating 

 the humors of the eye. 



736. It is probably on the same principle that we are to account for the re- 

 markable results obtained by Flourens (Op. cit.) from section of the portion of the 

 Auditory nerve proceeding to the Semicircular canals. Section of the horizontal 

 semicircular canal in Pigeons, on both sides, induces a rapid, jerking, horizontal 

 movement of the head from side to side ; and a tendency to turn to one side, 

 which manifests itself whenever the animal attempts to walk forwards. Section 

 of a vertical canal, whether the superior or inferior, of both sides, is followed by 

 a violent vertical movement of the head. And section of the horizontal and 

 vertical canals, at the same time, causes horizontal and vertical movements. 

 Section of either canal on one side only is followed by the same effect as when 

 the canal is divided on both sides; but this is inferior in intensity. The move- 

 ments continue to be performed during several months. In Rabbits, section of 

 the horizontal canal is followed by the same movements as those exhibited by 

 Pigeons ; and they are even more constant, though less violent. Section of the 

 anterior vertical canal causes the animal to make continued forward " somersets;" 

 whilst section of the posterior vertical canal occasions continual backward 

 " somersets." The movements cease when the animal is in repose ; and they 

 recommence when it begins to move, increasing in violence as its motion is more 

 rapid. These curious results are supposed by M. Flourens to indicate that the 

 nerve supplying the semicircular canals does not minister to the sense of hear- 

 ing, but to the direction of the movements of the animal; but they are fully ex- 

 plained upon the supposition that the normal function of the semicircular canals 

 is to indicate to the aninufl the direction of sounds, and that its movements are 

 partly determined by these ; so that a destruction of one or other of them will 

 produce an irregularity of movement (resulting, as it would seem, from a sort 

 of giddiness on the part of the animal), just as when one of the eyes of a bird 

 is covered or destroyed, as in the experiments previously cited. 



737. The numerous Experiments which have been made for the purpose of 

 determining the functions of the Thalami Optici and Corpora Striata, have not 

 yielded any very satisfactory results ; and this on account of the impossibility of 

 completely isolating them in such a manner as to limit the operation (whether 

 this be section, removal, or irritation) to them alone. Thus it is impossible to 



