386 THE PERIPHERAL NERVOUS SYSTEM 



stituting the anlage of the cochlear duct (Fig. 385 a). The wider, dorsal portion 

 of the otocyst is the vestibular anlage, which shows indications dorsally of the de- 

 veloping semicircular canals. These are formed in 11 mm. embryos as two 

 pouches — the anterior and posterior canals from a single pouch at the dorsal 

 border of the otocyst, the lateral canal later from a lateral outpocketing (Fig. 

 385 c). Centrally the walls of these pouches flatten and fuse to form epithelial 

 plates. In the three plates thus produced canals are left peripherally, commimi- 

 cating with the cavity of the vestibule. Soon the epithehal 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 be- 

 fore 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 coch- 

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

 posterior canals have a common opening dorsaUy into the vestibule, while their 

 opposite ends and the cranial end of the lateral canal are dilated to form ampulla. 

 In each ampulla is located an end organ, the crista ampullaris, which will be re- 

 ferred 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 sacculus, which is connected with the cochlear duct (Fig. 

 385 e,f). 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 £ach other, but each remains attached to the endolymph duct by 

 a slender canal which 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 epitheKum of the labyrinth at first is composed of a single layer of low 

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

 the epithehal cells in certain regions and these become modified to produce special 

 sense organs. These end organs are the crista ampullares in the ampuUae of the 

 semicircular canals, the macula acusticce in the utriculus and sacculus, and the 

 spiral organ (of Corti) in the cochlear duct. 



The cristse and macule are static organs, or sense organs for maintaining 

 equihbrium. In each ampulla, transverse to the long axis of the canal, the epi- 

 thelium and underlying tissue form a curved ridge, the crista. The cells of the 



