CII. XLIX.] 



SEMICIRCULAR CANALS 



707 



the bony canal is closest ; and the innermost layer is a somewhat 

 flattened epithelium. 



At the ampulla there is a different appearance; the tunica 

 propria is raised into a hillock called the crista acoustics (see fig. 513) ; 

 the cells of the epithelium become columnar in shape, and to some 

 of them fibres of the auditory nerve pass, arborising round them ; 

 fchese cells are provided with stiff hairs, which project into what is 

 called the cupula, a mass of mucus-like material containing otoliths 

 or crystals of calcium carbonate. Between the hair-cells are fibre- 

 cells which act as supports (fig. 514). When the endolymph in the 

 interior of the canals is thrown into vibration, the hairs of the hair- 

 cells are affected, and a nervous im- 

 pulse is set up in the contiguous 

 nerve-fibres, which carry it to the 

 central nervous system. 



The walls of the saccule and 

 utricle are similar in composition, 

 and each has a similar hillock, called 

 a macula, to the hair-cells on which 

 nerve-fibres are distributed. 



The macula of the utricle and 

 the cristae of the superior and hori- 

 zontal canals are supplied by the 

 vestibular division of the eighth or 

 auditory nerve. The macula of the 

 saccule and the crista of the posterior 

 canal are supplied by a branch of the 

 cochlear division of the same nerve 

 (see p. 643). 



When these canals are diseased 

 in man, as in Meniere's disease, 

 there are disturbances of equili- 

 brium : a feeling of giddiness, which may lead to the patient's fall- 

 ing down, is associated with nausea and vomiting. In animals 

 similar results are produced by injury, and the subject has been 

 chiefly worked out on birds by Flourens, where the canals are large 

 and readily exposed, and more recently in fishes, by Lee. 



Thus, if the horizontal canal is divided in a pigeon, the head is 

 thrown into a series of oscillations in a horizontal plane, which are 

 increased by section of the corresponding canal of the opposite side. 

 After section of the vertical canals, the forced movements are in a 

 vertical plane, and the animal tends to turn somersaults. 



"When the whole of the canals are destroyed on both sides 

 the disturbances of equilibrium are of the most pronounced character. 

 Goltz describes a pigeon so treated which always kept its head with 



FIG. 514. 1, Hair-cell ; 3, hair-cell, showing 

 the hair broken, and the base of the hair 

 split into its constituent fibrils ; 2, fibre- 

 cell; N, bundle of nerve-fibres which 

 have lost their medullary sheath, and 

 terminate by arborising round the base 

 of the hair-cells ; A.B., surface of tunica 

 propria. (After Retzius). 



