§82 A MANUAL OF VETERINARY PHYSIOLOGY 



These hair cells arise from definite areas of the utricle and 

 saccule, and project into a mass of mucus containing, especially 

 in the horse, fine solid particles of carbonate of lime, known as 

 otoliths. The vibrations of the endo-lymph set these hair cells 

 in motion, and so convey impulses to the brain. The vibra- 

 tions, however, are not sound vibrations, but such as are produced 

 by the movements of the head up and down, right and left, the 

 fluid in the semicircular canals concerned being readily influenced 

 by such movements. The displacement of the fluid impinges 

 on the nerve-endings, and starts impulses to the cerebellum. 

 By means of these impulses the brain is informed of the position 

 of the body, more particularly, perhaps, movements of rotation, 

 and co-ordination becomes possible under the influence of the 

 cerebellum. There are other effects upon voluntary muscle ; 

 it is supposed that from the labyrinth a constant outflow of 

 impulses is occurring, which maintains them in a condition of 

 tone, for it can be shown experimentally that the destruction of 

 the semicircular canals causes the loss of muscular tone on that 

 side, and in case of death delays the appearance of rigor mortis. 

 Among their other functions, the semicircular canals may be re- 

 garded as muscle-tone organs. The functions of the utricle and 

 saccule are probably connected with the position of the body 

 when at rest, and of forward movement not associated with 

 orientation. In these organs the hair cells are supposed to be 

 affected by actual mechanical stimulation by the otoliths in their 

 vicinity. 



The function of the labyrinth, to quote the words of Sherring- 

 ton, is to keep the world right side up for the organism, by 

 keeping the organism right side up to the external world. It is 

 by means of the semicircular canals that an animal is made 

 acquainted with the direction in which its body is travelling, 

 forward or backward, right or left, up hill or down, or in the 

 movements which occur in jumping. Another function connected 

 with the semicircular canals is that of the control of the ocular 

 muscles and the maintenance of a horizontal pupil. It is no 

 wonder, considering their extraordinary importance, that they 

 are securely lodged within the substance of the hardest bone in 

 the body. 



Stimulation of the semicircular canals produces giddiness and 

 movements of the eyes. If the membranous canals be destroyed 

 in a pigeon, the resulting phenomena depend upon the position 

 of the canal which has been injured. If the horizontal canal, the 

 head oscillates in a horizontal plane ; if the vertical canals be 

 damaged, forced movements occur in a vertical plane ; standing 

 and locomotion become impossible. If all three canals be 

 destroyed, violent inco-ordinate movements result, the animal 



