THE PHYSIOLOGY OF THE BRAIN. 613 



much more complicated reflex acts than one with its spinal 

 cord alone. 



The Cerebellum and Pons Varolii. (Figs. 74, 75). The 

 anterior part of the medulla oblongata is covered above by 

 the cerebellum and below by the pons, the latter of which is 

 mainly a transverse commissure uniting the hemispheres of 

 the cerebellum, though the pyramidal and other longitudinal 

 commissural fibres run through it ; and in it are many gray 

 nuclei. The halves of the cerebellum are also united with 

 one another by transverse fibres of its middle lobe ; and, be- 

 hind, by the posterior peduncles with the restiform bodies 

 and the medulla, and, in front, by the anterior peduncles, 

 with the cerebrum. Besides its gray surface with small nerve- 

 cells and the cells of Purkinje (Fig. 82) it contains other 

 more central gray matter. The most striking anatomical fact" 

 in relation to the cerebellum is its close connection with the 

 afferent tracts of the spinal cord, nearly all of which except 

 the fibres of the fillet are only connected with the cerebrum 

 through the intervention of the cerebellum. The same is true 

 of the vestibular portion of the auditory nerve and probably 

 also of most of the afferent fibres of all the posterior cranial 

 nerves. The cerebellum is thus subjected to influences from 

 many regions of the Body ; the skin, the muscles, the ears, and 

 probably also the eyes are sources of impulses streaming into 

 it all the time, and modifying the conditions of its gray matter 

 and the nature of the impulses in turn issued from that. The 

 most marked result of extensive injury of the cerebellum is 

 muscular inco-ordination ; it seems to be a chief organ of what 

 we may call personally acquired reflexes, as distinguished from 

 inherited. 



Every one has to learn to stand, walk, run, and so on; at 

 first all are difficult, but after a time become easy and are 

 performed unconsciously. In standing or walking very many 

 muscles are concerned, and if the mind had all the time to 

 look directly after them we could do nothing else at the same 

 time; we have forgotten how we learnt to walk, but in ac- 

 quiring a new mode of progression in later years, as skating, 

 we find that at first it needs all our attention, but when once 

 learnt we have only to start the series of movements and they 

 are almost unconsciously carried on for us. At first we had 

 to learn to contract certain muscle groups when we got par- 

 ticular sensations, either tactile, from the soles, or muscular, 



