384 



THE PERIPHERAL NERVOUS SYSTEM 





sacculus and utriculus are more completely separated, the canals are relatively 

 longer, their ampullae more prominent and the cochlear duct is coiled about two 

 and a half turns (Fig. 366 /). In the adult, the sacculus and utriculus become 

 completely separated from each other, but each remains attached to the endo- 

 lymph duct by a slender canal which represents the prolongation of their re- 

 spective walls. Similarly, the cochlear duct is constricted from the sacculus, 

 the basal end of the former becomes a blind process and a canal, the ductus 

 reuniens, connects the cochlear duct with the sacculus. 



The epithelium of the labyrinth at first is composed of a single layer of low 

 columnar cells. At an early stage, fibers from the acoustic nerve grow between 

 the epithelial cells in certain regions and it becomes modified to produce special 

 sense organs. These end organs are the cristtz acusticce in the ampullae of the 

 semicircular canals; the macula acustica in the utriculus and sacculus, and the 

 spiral organ (of Corti) in the cochlear duct. 



The cristae and maculae are static organs, or sense organs for equilibrium. In 

 each ampulla transverse to the long axis of the canal the epithelium and under- 

 lying tissue form a curved ridge, the crista. The cells of the epithelium are 

 differentiated: (i) Into sense cells with bristle-like hairs at their ends, and (2) 

 into supporting cells. About the bases of the sensory cells branch nerve fibers 

 from the vestibular division of the acoustic nerve. The maculae resemble the 

 cristae in their development save that larger areas of the epithelium are differ- 

 entiated into cushion-like end organs. Over the maculae concretions of lime salts 

 may form otoconia which remain attached to the sensory bristles. The true or- 

 gan of hearing, the spiral organ, is developed in the basal epithelium of the coch- 

 lear duct, basal having reference here to the base of the cochlea. The develop- 

 ment of the spiral organ has been studied carefully only in the lower mammals, 

 in the pig by Shambaugh, Hardesty and Prentiss. In pig embryos of 5 cm. the 

 basal epithelium is thickened, the cells becoming highly columnar and the nuclei 

 forming several layers. In later stages, 7 to 9 cm., inner and outer epithelial 

 thickenings are differentiated, the boundary line between them being the future 

 spiral tunnel (Fig. 367 A). At the free ends of the cells of the epithelial swellings 

 there is differentiated a cuticular structure, the membrana tectoria, which appears 

 first in embryos of 4 to 5 cm. The cells of the inner (axial) thickening give rise to 

 the epithelium of the spiral limbus, to the cells lining the internal spiral sulcus and 

 to the supporting cells and inner hair cells of the spiral organ (Fig. 367 B, C). The 

 outer epithelial thickening forms the pillars of Corti, the outer hair cells and sup- 

 porting cells of the spiral organ. Differentiation begins in the basal turn of the 



