THE ORGANS OF SPECIAL SENSE 589 



numerous fibres radiate to the bony spiral laminae, in the bases of 

 which they enter the spiral ganglia (Fig. 389, p). 



The cells of the spiral ganglia are peculiar, in that while of the 

 same general type as the spinal ganghon cell they maintain their 

 embryonic bipolar condition (see page 425) throughout life. Their 

 axones follow the already described course through the modiolus and 

 thence through the internal auditory meatus to their terminal nuclei 

 in the medulla (see page 499). Their dendrites become medul- 

 lated like the dendrites of the spinal ganglion cells and pass outward 

 in bundles in the bony spiral lamina (Fig. 389, 0, and Fig. 390). 

 From these are given off branches which enter the tympanic portion 

 of the lamina, where they lose their medullary sheaths and pass 

 through the foramina nervosa (minute canals in the tympanic part 

 of the spiral lamina) to their terminations in the organ of Corti. 

 In the latter the fibres run in three bundles parallel to Corti 's tun- 

 nel. One bundle lies just inside the inner pillar beneath the inner 

 row of hair cells (Fig. 390). A second bundle runs in the tunnel to 

 the outer side of the inner pillar (Fig. 390). The third bundle crosses 

 the tunnel (tunnel-fibres) and turns at right angles to run between 

 the cells of Deiter beneath the outer hair cells (Fig. 390). From all 

 of these bundles of fibres are given off delicate terminals which end 

 on the hair cells. (For details of acoustic tract see pp. 481, 482, 

 and 499.) 



Development of the Ear 



The essential auditory part of the organ of hearing, the membranous laby- 

 rinth, is of ectodermic origin. This first appears as a thickening followed by 

 an invagination of the surface ectoderm in the region of the posterior cerebral 

 vesicle. This is known as the auditory pit. By closure of the lips of this pit 

 and growth of the surrounding mesodermic tissue is formed the otic vesicle or 

 otocyst, which is completely separated from the surface ectoderm. Diver- 

 ticula soon appear passing off from the otic vesicle. These are three in number 

 and correspond respectively to the future endolymphatic duct, the cochlear 

 duct and the membranous semicircular canals. Within the saccule, utricle, 

 and ampulla special differentiations of the lining epithelium give rise to the 

 maculae and cristae acusticas. Of the cochlear duct the upper and lateral walls 

 become thinned to form Reissner's membrane and the epithelium of the outer 

 wall, while the lower wall becomes the basilar membrane, its epithelium under- 

 going an elaborate specialization to form the organ Corti. 



Of the cochlea, only the membranous cochlear duct develops from the otic 

 vesicle; the scala vestibuli, scala tympani, and bony cochlea developing from 

 the surrounding mesoderm. The mesodermic connective tissue at first com- 



