CH. XLIX.] 



LABYKINTHINE IMPEESSIONS 



705 



2. Muscular impressions. Quite as important as the tactile sense 

 from the skin is the muscular sense, the sense which enables us to 

 know what we are doing with our muscles. We have hitherto chiefly 

 spoken of the muscular nerves as being motor; they also contain 

 sensory fibres ; these 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 cerebellum and 

 the Eolandic area. In some Ceases of locomotor ataxy there is but 

 little loss of tactile sensibility, and the condition of inco-ordination 

 is then chiefly due to the loss of the 



muscular sense. 



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. 



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. Here, how- 

 ever, we must pause to consider first some anatomical facts in 

 connection with the semicircular canals that make up the labyrinth. 

 Fig. 511 is an external view of the internal ear ; it is enclosed within 

 the petrous portion of the temporal bone; and consists of three 

 parts the vestibule (1), the three semicircular canals (3, 4, 5) which 

 open into the vestibule, and the tube, coiled like a snail's shell, called 

 the cochlea (6, 7, 8). The cochlea is the part of the apparatus which 

 is concerned in the reception of auditory impressions ; it is supplied 

 by the cochlear division of the eighth or auditory nerve. The 



2 Y 



FIG. 511. 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, fenestra rotunda. 

 The smaller figure in outline below 

 shows the natural size. (Sommering.) 



