384 HISTOLOGY. 



of the malleus lying near it becomes imbedded in its mesenchymal layer, 

 and its inner entodermal layer is made by the expansion of the tympanic 

 cavity. The enlargement of the tympanic cavity continues after birth 

 when jt invades the spaces formed within the mastoid part of the temporal 

 bone. 



In spite of these modifications the course of the spiracular cleft is 

 retained in the adult. The ectodermal depression and its surrounding 

 elevations constitute the external ear; the pharyngeal outpocketing persists 

 as the auditory tube and the tympanic cavity of the middle ear. It opens 

 freely into the pharynx and contains air. 



SACCULUS, UTRICULUS, AND SEMICIRCULAR DUCTS. 



The walls of all these structures consist of three layers. On the out- 

 side there is connective tissue with many elastic fibers and occasional pig- 

 ment cells. This is followed by a narrow basement membrane said to 

 form small nodular elevations toward the third and innermost layer, 

 the simple flat epithelium. Near the maculae and cristae the connective 

 tissue and the basement membrane become thicker; the epithelial cells 

 are columnar with a cuticular border. In the neuro-epithelium of these 

 areas there are two sorts of cells, sustentacular and hair cells. The 

 sustentacular or fiber cells extend clear across the epithelium and are some- 

 what expanded at both ends; they contain oval nuclei. Hair cells, which 

 receive the stimuli, are columnar cells limited to the 

 superficial half of the epithelium; they have large 

 spherical nuclei near their rounded basal ends, and a 

 clump of fine agglutinated filaments projecting from 

 ^ ^ their free surface. The nerves lose their myelin as 



p& they enter the epithelium and ascend to the bases of 

 FIG. 434. OTOCONIA the hair cells. There they bend laterally, forming a 



FROM THE SACCU- j 1 I 1 1 I ' 



LUS OF AN INFANT, dense network which appears as a granular layer in or- 

 dinary preparations; the granules are optical sections 

 and varicosities. The horizontal fibers terminate like their occasional 

 branches, by ascending between the hair cells, on the sides of which they 

 form pointed free endings. They do not reach the free surface of the 

 epithelium. This surface is covered by a continuation of the cuticula, 

 a "membrana limitans," which is perforated by the hairs. Over the two 

 maculae there is a soft substance containing very many crystals of calcium 

 carbonate, 1-15 jj. long, which are named otoconia. .(Large "ear stones" 

 of fishes are called otoliths.) Over the cristae of the semicircular ducts 

 there is a gelatinous substance, transparent in fresh preparations, but 

 coagulated and rendered visible by reagents. 





