FUNCTIONS OF THE SENSORY GANGLIA. 521 



which, in function, the corpora geniculata are not distin- 

 guished), are the homologues of the optic lobes in birds, 

 Amphibia and fishes, and may be regarded as the prin- ' 

 cipal nervous centres for the sense of sight. The experi- 

 ments oT Flourens, Longet, and Hertwig, show that 

 removal of the corpora quadrigemina wholly destroys the | 

 power of seeing ; and diseases in which they are disor- \ 

 ganized are usually accompanied with blindness. Atrophy \ 

 of them is also often a consequence of atrophy of the eyes. \ 



Destruction of one of the corpora quadrigemina (or of j 

 one optic lobe in birds), produces blindness of the oppo- 

 site eye. 



This loss of 'sight is the only apparent injury of 

 sensibility sustained by the removal of the corpora quad- 

 rigemina. The removal of one of them affects the move- 

 ments of the body, so that animals rotate, as after division 

 of the crus cerebri, only more slowly : but this is probably 

 due to giddiness and partial loss of sight. The more 

 evident and direct influence is that produced on the iris. 

 It contracts when the corpora quadrigemina are irritated : 

 it is always dilaled when they are removed : so that they 

 may be regarded, in some measure at least, as the nervous 

 centres governing its movements, and adapting them to 

 the impressions derived from the retina through the optic 

 nerves and tracts. 



Concerning the functions, taken as a whole, discharged by 

 the olfactory and optic lobes, the grey substance of the pons, 

 the corpora striata and optic thalami (b, d, fig. 146), with 

 some other centres of grey matter not so distinct, such as the 

 grey matter on the floor of the fourth ventricle with which 

 the auditory nerve is connected, the most philosophical 

 theory is undoubtedly that which has been so ably enun- 

 ciated by Dr. Carpenter. He supposes these ganglia to 

 constitute the real sensorium ; that is to say, it is by means 

 of them that the mind becomes conscious of impressions 



