THE CEREBELLUM. 525 



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 

 and involuntary, anyone may convince himself by trying 

 to perform each individual movement concerned, strictly 

 as a voluntary act. 



That such movements are reflex and essentially inde- 

 pendent 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 from the cerebrum, as for impressions 

 carried upivards to them by the different nerves which 

 preserve their connection with the organs of the various 

 senses. 



STRUCTURE AND PHYSIOLOGY OF THE CEREBELLUM. 



The cerebellum (7, 8, 9, 10, fig. 138) is composed of an 

 elongated central portion called the vermiform processes, 

 and two hemispheres. Each hemisphere is connected with 



k is the valve of Yieussens, which has been divided so as to expose the 

 fourth ventricle ; J, hippocampus major and corpus fimbriatum, or tsenia 

 hippocampi ; in, hippocampus minor ; n, eminentia collaterals ; o, fourth 

 ventricle ; p, 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. 



