752 



FUNCTIONS OF THE CEKEBELLUM 



[CH. LI. 



enables us to know what we are doing with our muscles. Sensory 

 fibres pass from the muscles, and their tendons to the posterior roots 

 of the spinal nerves, and the impulses ascend the sensory tracts 

 through cord -and brain to reach the ascending parietal convolution. 

 Their offshoots, which carry the non-sensory impulses to the cere- 

 bellum, reach it via Clarke's column and the cerebellar tracts. In 

 many cases of locomotor ataxy there is but little loss of tactile 

 sensibility, and the condition of incoordination is then chiefly due to 



the loss of impressions from motorial 

 organs (muscles and joints). 



3. Visual impressions. The use of 

 visual impressions in guiding the 

 nervous centres for the maintenance 

 of equilibrium is seen in those cases 

 of locomotor ataxy where there is loss 

 of equilibrium when the patient closes 

 his eyes. Destruction of the eyes in 

 animals often causes them to spin 

 round and lose their balance. The 

 giddiness experienced by many people 

 on looking at moving water, or after 

 the onset of a squint, or when objects 

 are viewed under unusual circum- 

 stances, as in the ascent of a mountain 

 railway, is due to the same thing. The 

 importance of keeping one's eyes open 

 is brought home to one very forcibly 

 when one is walking in a perilous posi- 

 tion, as along the edge of a precipice, 

 where an upset of the equilibrium 

 would be attended with serious con- 

 sequences. Under more ordinary cir- 

 cumstances, the non-sensory visual offshoots which enter the cere- 

 bellum are sufficient to maintain equilibrium. In speaking of visual 

 impressions it should be understood that these in themselves are not 

 the actual guide. It is the projection of what is seen in relation to 

 the position of the body (ascertained by the innervation of the head 

 muscles and ocular muscles) that is the chief guide. 



4. Labyrinthine impressions. These are the most important of 

 all ; they are the impressions that reach the central nervous system 

 from that part of the internal ear called the labyrinth, and in this 

 case the sensory element is subordinate to the non-sensory. Here, 

 however, we must pause to consider some anatomical facts in 

 connection with the semicircular canals that make up the labyrinth. 

 Fig. 460 is an external view of the internal ear ; it is enclosed within 



FIG. 460. Right bony labyrinth, viewed 

 from the outer side. The specimen 

 here represented was prepared by 

 separating piecemeal the looser sub- 

 stance of the petrous bone from the 

 dense walls which immediately en- 

 close the labyrinth. 1, the vestibule; 

 2, fenestra ovalis; 3, superior semi- 

 circular canal ; 4, horizontal or ex- 

 ternal canal ; 5, posterior canal ; *, 

 ampullae of the semicircular canals ; 

 6, first turn of the cochlea ; 7, second 

 turn; 8, apex; 9, feriestra rotunda. 

 The smaller figure in outline below 

 shows the natural size. (Sommering.) 



