FUNCTIONS OF THE SENSORY GANGLIA. 523 



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 (J, d, fig. 139), 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 

 made on the organs or tissues with which (by means of 

 nerve-fibres) they are in communication. Thus impres- 

 sions made on the optic nerve, or its expansion in the 

 retina, are conducted by the fibres of the optic nerve to 

 the corpora quadrigemina, and through the medium of 

 these ganglia the mind becomes conscious of the impres- 

 sion made. And impressions on the filaments of the 

 olfactory or auditory nerve are in the same way perceived 

 through the medium of the olfactory or auditory ganglia, 

 to which they are first conveyed. The optic thalami and 

 corpora striata probably have some function of a like kind 

 perhaps in relation to ordinary sensation, but nothing is 

 certainly known regarding them. 



Besides their functions, however, as media of communi- 

 cation between the mind and external objects, these sensory 

 ganglia, as they are termed, are probably the nerve-centres 

 by means of which those reflex acts are performed which 

 require either a higher combination of muscular acts than 

 can be directed by means of the medulla oblongata or 

 spinal cord alone, or on the other hand, such reflex actions 

 as require for their right performance the guidance of 

 sensation. Under this head are included various acts, as 

 walking, reading, writing, and the like, which we are 



