148 EAR 



into the saccule, forming the only permanent means of communication between 

 them. Another fold, or constriction, appears presently, somewhat lower down 

 and converts the connexion between the saccule and the cochlea into the narrow 

 duct of Hensen (canalis re-uniens). 



In the meantime the cochlea-rudiment at the ventral end of the now labyrinthine 

 vesicle becomes elongated into a tube, which, as it grows, becomes coiled upon 

 itself in such a manner as to produce the spiral structure of this part of the auditory 

 organ (figs. 188, 189). This coiling, however, only occurs in mammals ; in birds, 

 the cochlea is a short straight blind tube. 



All these parts of the labyrinth are, when first formed, simple epithelial tubes 

 surrounded by and imbedded in embryonic connective tissue. As development 

 proceeds, and the skull begins to form, a cartilaginous capsule becomes developed 

 around the several parts of the labyrinth, and this at length becomes ossified. 

 The cartilaginous capsule does not closely invest the epithelial structures ; they 

 are immediately surrounded by embryonic connective tissue, which forms an 

 internal periosteal lining to the capsule and a special covering to the epithelial 

 tube. These two connective-tissue membranes are everywhere separated from 

 one another by gelatinous connective tissue, composed of semi-fluid ground sub- 

 stance and branching corpuscles, except along one border, where they are in 

 continuity. But in the cochlea the gelatinous tissue is above and below the 

 epithelial tube, the place of the modiolus being occupied by embryonic tissue 

 which is not gelatinous, and is connected with that lining the capsule by similar 

 non-gelatinous tissue separating the turns of the cochlea from one another, and 

 also running in the position of the future spiral lamina. 



The bone, which is formed by ossification of the cartilaginous capsule, is of a spongy nature, 

 but it becomes coated internally by layers of compact bone deposited by the periosteal 

 lining. The modiolus and septa of the cochlea, as well as the osseous spiral lamina, are formed 

 wholly in connective tissue without any preformation in cartilage. 



The perilymphatic spaces throughout the whole labyrinth are produced by a gradual vacuo- 

 lation and disappearance of the gelatinous tissue which surrounds the membranous labyrinth. 

 In the cochlea this conversion into perilymph begins in the proximal turn of the spiral and 

 extends hence towards the distal end. It is only with the development of these perilymph- spaces 

 (scalae) that the cochlear tube, which was previously oval in section, acquires the characteristic 

 triangular section which we see in the fully formed organ. 



The cells which form the wall of the epithelial tube become variously modified in different 

 parts of" the labyrinth to produce the characteristic structures which there occur viz. the 

 hair-cells, the rods of Corti, the sustentacular cells of Deiters, and the epithelium lining the 

 labyrinth. The membrana tectoria appears as a cuticular deposit over the columnar cells 

 which are becoming developed into the organ of Corti. 



The auditory nerve arises from a ganglionic mass which is early divided into 

 an acoustic and a facial portion (geniculate ganglion) (fig. 186). The acoustic 

 ganglion lies on the front edge of the auditory vesicle with its lower end turning on 

 to its mesial aspect (figs. 186, 187). It consists (in embryos of 7 mm., twenty-sixth 

 day) of an upper and a lower part. The central root of the ganglion springs from 

 the upper part, and each division has its own peripheral branches. According to 

 Streeter's researches, the lower part is not the cochlear ganglion as described by 

 W. His, Jr. The ganglion spirale develops from the ventral border of the pars inferior, 

 becomes coiled with the cochlea, isolated from the rest of the common ganglion, and 

 connected secondarily with the neural tube by a separate nerve-root, the cochlear 

 root. Thus the pars superior and pars inferior together constitute the vestibular 

 ganglion. From its upper portion are derived the nerves to the utricle and to 

 the ampullae of the superior and lateral canals, while from the lower portion come 

 the nerves to the saccule and ampulla of the posterior semicircular canal (fig. 189). 



