CH. XLIX.] 



SEMICIRCULAR CANALS. 



685 



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 guid- 

 ing 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 cir- 

 cumstances, 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 position, as along the edge 

 of a precipice, where an upset 

 of the equilibrium would be at- 

 tended with serious consequences. 



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, however, we must pause 

 to consider first some anatomical 

 facts in connection with the semi- 

 circular canals that make up the 

 labyrinth. Fig. 508 is an ex- 

 ternal view of the internal ear ; 

 it is enclosed within the petrous 

 portion of the temporal bone ; 



and consists of three parts the vestibule (i), the three semi- 

 circular canals (3, 4, 5) which open into the vestibule, and the 

 tube, roiled like a snail's shell, called the cochlea (6, 7, 8). The 

 cochlea is the part of the ajijiiinitus which is concerned in the 

 reception of auditory impressions ; it is supplied by the cochlear 

 division of the eighth rr auditory nerve. The remainder of the 

 internal ear is concerned not in hearing, but in the reception of 

 the impressions we are now studying. Within the vestibule are 

 two chambers made of membrane, called the utricle and the 

 saccule ; these communicate with one another and with the canal 



Fig. 508. Right bony labyrinth, viewed 

 from the outer side. The specimen here 

 represented was prepared by sepa- 

 rating piecemeal the looser substance 

 of the petrous bone from the dense 

 walla Which immediately enclose the 

 labyrinth, i, the vestibule ; 2, fen- 

 estra ovalis ; 3, superior semicircular 

 canal ; 4, horizontal or external canal ; 

 5, posterior canal ; *, ampullae of the 

 semicircular canals ; 6, first turn of 

 the cochlea ; 7, second turn ; 8, apex ; 

 o, fenestra rotundn. The smaller 

 figure in outline below shows the 



natural size. 2 * (Summering.) 



