CH. XLIX.] 



SEMICIRCULAR CANALS. 



687 



canal consists of three layers, the outer of which is fibrous and 

 continuous with the periosteum that lines the bony canal ; then 

 comes the tunica pro/>ria, composed of homogeneous material, and 

 thrown into papillae except just where the attachment of the 

 membranous to 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 acoustica (see 

 fig. 510) ; the cells of the epithelium become columnar in shape, 

 and to some of them fibres of the auditory nerve pass, arborising 

 round them ; these 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. 

 511). When the endolymph in 

 the interior of the canals is 

 thrown into vibration, the hairs 

 of the hair-cells are affected, and 

 a nervous impulse 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 composi- 

 tion, 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 cristse of the superior and 

 horizontal 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. 719). 



When these canals are diseased in man, as in Meniere's disease, 

 there are disturbances of equilibrium : a feeling of giddiness, which 

 may lead to the patient's falling down, is associated with nausea 

 and vomiting. In animals similar results are produced by injury, 

 ami the subject has been chiefly worked out on birds by Flourens, 

 where the canals are large and readily ex posed, and more recently 

 in fishes, by Lee. 



Fig. 511. i, hair-cell; 3, hair-cell, show- 

 ing: 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.) 



