502 EMBRYOLOGY. 



the perilymphatic spaces of the semicircular canals, and (2) a bony 

 envelope (KL 1 ) of the atriuin or vestibuluna, which constitutes the 

 middle region of the bony labyrinth. 



The envelope of the epithelial cochlear duct, which becomes the 

 bony cochlea with its scalse, undergoes a more complicated alteration. 

 It is already differentiated, at the time when the duct (fig. 279 dc) 

 makes only half of a spiral turn, into an inner, soft and an outer, 

 firm layer, the latter becoming cartilage (kk). The cartilaginous 

 capsule (fig. 281 kk), which is continuous with the cartilaginous 

 mass of the remaining parts of the labyrinth and together with them 

 constitutes a part of the os petrosum, afterwards encloses a lenticular 

 cavity and possesses below a broad opening, through which the coch- 

 lear nerve (nc) enters. The resemblance to a snail-shell is not yet 

 observable ; it takes place gradually and is produced by two changes : 

 by the outgrowth of the epithelial duct and by the differentiation of 

 the soft tissue surrounding it into parts which are fluid and such as 

 become more firm. 



In its outgrowth the epithelial ductus cochlearis describes within 

 its capsule the previously mentioned spiral turns (dc), shown in cross 

 section in fig. 283 ; at the same time it remains quite closely approxi- 

 mated to the inner surface of the capsule (kk). The cochlear nerve 

 (nc) ascends from its place of entrance straight up through the 

 centre of the turns, consequently in the axis of the capsule, and 

 gives off numerous lateral branches to the concave side of the 

 cochlear duct (dc), where they are enlarged into the ganglion 

 (gsp), which has now also grown out into a spiral band. The 

 nutritive blood-vessels have taken the same course as the nerves. 



When the development has advanced as far as this, there still 

 remains to be accomplished only an histological differentiation in 

 the soft mesenchyme which fills the cartilaginous capsule in order to 

 produce the parts of the finished cochlea that are still wanting the 

 modiolus, the lamina spiralis ossea, the bony cochlea, and the vesti- 

 bular and tympanic scalse (fig. 283). Here, as in the vicinity of the 

 semicircular canals the utriculus and the sacculus, the mesenchyme 

 is differentiated into a firmer connective substance, which becomes 

 fibrous, and into a gelatinous tissue (g), which is continually becoming 

 softer. Fibrous connective substance is developed first around the 

 trunks of the nerves (nc) and blood-vessels that enter the cartilaginous 

 capsule ; furnishing the foundation of the future bony axis of the 

 snail-shell (M ), secondly it furnishes an envelope for nerve-fibres (N) 

 that run frotu the axis to the epithelial cochlear duct, for the gangli- 



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