CHAP. XI.] FUNCTIONS OF THE COKPORA QUADRIGEMINA. 353 



nerves remain uninjured, the iris continued to respond to the 

 stimulus of light, but the total removal of the tubercles paralysed 

 the irides. If the lobes of the brain and cerebellum were removed, 

 leaving the tubercles untouched, the irides would continue to con- 

 tract. These experiments leave no room to doubt that the optic 

 tubercles are the encephalic recipients of the impressions necessary to 

 vision, which doubtless are simultaneously felt by means of the optic 

 thalami; and that they are the centres of those movements of the 

 iris which contribute largely not only to protect the retina, but like- 

 wise to increase the perfection of vision. The optic nerve is. at once 

 the nerve of vision, and the excitor of motor impulses which are 

 conveyed to the iris by the third nerve, which takes its origin very 

 near to the optic tubercles. It is interesting to add, that irritation 

 of an optic tubercle on one side causes contraction of both irides: 

 this is quite in accordance with the well-established fact, that, if 

 light be admitted to one eye so as to cause contraction of its pupil, 

 the other pupil will contract at the same time. So simultaneous is 

 the action of the two centres; so rapid must be the transmission of 

 the stimulus from one side to the other. 



When the injuries inflicted on these tubercles were deep, more or 

 less general convulsive movements were produced ; if one tubercle 

 were injured, the opposite side only was so affected. These con- 

 vulsions were due to the lesion of the central parts of the medulla 

 oblongata, with which the optic tubercles are intimately con- 

 nected. A remarkable vertiginous movement was likewise caused, 

 the animal turning to the side from which the tubercle had been 

 removed. It does not appear that this rotation could be attri- 

 buted to any special influence of the medulla oblongata, but 

 rather to a state of vertigo induced by the partial destruction of 

 vision ; for Flourens found that the same effects could be produced 

 in pigeons by blindfolding one eye. The movements, however, were 

 not so rapid, nor did they continue so long. And Longet saw the 

 same movements in pigeons in which he had evacuated the humours 

 of one eye.* 



It may be remarked, that deep injuries to the quadrigeminal 

 tubercles are very likely to affect the only commissural connexion 

 between the cerebrum and cerebellum (processus cerebelli ad testes), 

 the integrity of which must doubtless be essentially necessary to 

 ensure harmony of action between these two great nervous 

 centres. 



* Flourens' experiments have been amply confirmed by those of Hertwig 

 and Longet. 



VOL. I. A A 



