SENSORY GANGLIA. 413 



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 com- 

 munication. Thus impressions 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 

 impression 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 communica- 

 tion 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, writ- 

 ing, and the like, which we are accustomed to consider volun- 

 tary, 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, per- 

 formed under the guidance of the spinal cord or medulla 

 oblongata alone, we call simple reflex actions. It is true that, 

 in the performance 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 

 and involuntary, any one may convince himself by trying to 

 perform each individual movement concerned, strictly as a 

 voluntary act. 



That such movements are reflex and essentially independent 

 as regards their mere production of the will, there is no 

 doubt ; that the nerve-centres through which such reflex 

 actions are performed are the so-called sensory ganglia, is, of 

 course, only a theory which may or not be confirmed by future 

 investigations. 



Besides their possible functions in the manner just men- 

 tioned, it is supposed that these sensory ganglia may be the 

 means of transmitting the impulses of the will to the muscles, 

 which act in obedience to it, and thus be the centres of reflex 

 action as well for impressions conveyed downwards to them 



35 



