THE CEREBELLUM. 523 



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. U.-ider this head are included various acts, as 

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

 accustomed to consider voluntary, but which really are as 

 incapable of being performed by distinct and definite acts 

 of the will as are those more simple movements of which 

 we are not conscious, and which, performed under the 

 guidance of the spinal cord or medulla oblongata alone, 

 we call simple reflex actions. It is true that in the per- 

 formance of such acts as those just-mentioned, a certain 

 exercise of the will is required at the commencement, but 

 that the carrying out of its mandates is essentially reflex 



major and corpus finibriatum, or tsenia hippocampi ; m, hippocampus 

 minor ; n, eminentia collateralis ; o, fourth ventricle ; ^>, posterior 

 surface of medulla oblongata ; r, section of cerebellum ; s, upper part 

 of left hemisphere of cerebellum exposed by the removal of part of the 

 posterior cerebral lobe. 



