300 THE PERIPHERAL NERVOUS SYSTEM 



Soon, the epithelial plates are resorbed, leaving the semicircular canals as 

 in Fig. 385 d, e. Dorsally, a notch separates the anterior and posterior 

 canals. Of these canals, the anterior is completed before the posterior. 

 The lateral canal is the last to develop. 



In a 20 mm. embryo (Fig. 385 e) the three canals are present and the 

 cochlear duct has begun to coil like a snail shell. It will be seen that the 

 anterior and posterior canals have a common opening dorsally into the 

 vestibule, while their opposite ends, and the cranial end of the lateral canal, 

 are dilated to form ampullae. In each ampulla is located an end organ, the 

 crista ampullaris, which will be referred to later. By a constriction of its 

 wall the vestibule is differentiated into a dorsal portion, the utriculus, to 

 which are attached the semicircular canals, and a ventral portion, the sac- 

 culus, connected with the cochlear duct (Fig. 385 e, /). At 30 mm. the 

 adult condition is nearly attained. The 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. 385 /). In the adult, the sacculus and utriculus become completely 

 separated from each other, but each remains attached to the endolymph 

 duct by a slender canal that represents the prolongation of their respective 

 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, alone connects the two. 



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 these become modified 

 to produce special sense organs. These end organs are the cristce ampullares 

 in the ampullae of the semicircular canals, the maculcs acusticcs 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 main- 

 taining equilibrium. In each ampulla, transverse to the long axis of the 

 canal, the epithelium and underlying tissue form a curved ridge, the crista. 

 The cells of the epithelium are differentiated into: (i) sense cells, with 

 bristle-like hairs at their ends; and (2) 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 differentiated into cushion-like end 

 organs. Over the maculae, concretions of lime salts may form otoconia 

 which remain attached to the sensory bristles. 



The true organ of hearing, the spiral organ, is developed in the basal 

 epithelium of the cochlear duct, basal having reference here to the base 

 of the cochlea. The development of the spiral organ has been studied 

 carefully only in the lower mammals. According to Prentiss (1913), in 



