4OO 



NORMAL HISTOLOGY. 



capsule and the ectodermic canal. Within this layer clefts appear, 

 which gradually extend until two large spaces bound the mem- 

 branous cochlea above and below. 



These spaces, the scala vestibuli and the scala tympani, are 

 separated for a time from the scala media by a robust septum 

 consisting of a mesodermic layer of considerable thickness and the 

 wall of the ectodermic tube. With the further increase in the 



dimensions of the lymph-spaces the par- 

 titions separating them from the cochlear 

 duct are correspondingly reduced, until, 

 finally, the once broad layers are rep- 

 resented by frail and attenuated struct- 

 ures, the membrane of Reissner and 

 the basilar membrane, which con- 

 sequently include an ectodermic 

 stratum, the epithelial layer, strength- 

 ened by a mesodermic lamina, rep- 

 resented by the substantia propria and 

 its endothelioid covering. 



The main sac of the otic vesicle from 

 which the foregoing diverticula arise 

 constitutes the primitive membra- 

 nous vestibule, and later subdivides 

 into the saccule and the utricle. 

 This separation begins as an annular 

 constriction of the primitive vestibule, incompletely dividing the 

 vesicle into two compartments ; the ductus endolymphaticus 

 unites with the narrow canal connecting these vesicles in such man- 

 ner that each space receives one of a pair of converging limbs, an 

 arrangement foreshadowing the permanent relations of the parts. 



Even before the subdivision of the primitive vestibule is established 

 the vestibular end of the cochlear canal becomes constricted, so 

 that communication between this tube and the future saccule is main- 

 tained by only a narrow passage, the canalis reuniens. The de- 

 velopment of the maculae acusticas of the saccule and the utricle 

 depends upon the specialization of the epithelium within certain 

 areas associated with the distribution of the auditory nerves. The 

 nerve-fibres form their ultimate relations with the sensory areas by 

 secondary growth into the epithelial structures. 



From the foregoing it is apparent that the membranous laby- 

 rinth is genetically the oldest part of the internal ear, and that it 

 is in fact only the greatly modified and specialized closed otic vesi- 

 cle surrounded by secondary mesodermic tissues and spaces. 



The middle ear is derived from the expanded and metamor- 



Section through developing cochlea 

 of twenty-one-day rabbit embryo : e, 

 sections of ectodermic cochlear duct, or 

 scala media, surrounded by delicate mes- 

 odermic tissue (tn), within which large 

 lymph-spaces later appear; c, con- 

 densed cartilaginous capsule ; n, bundles 

 of nerve-fibres. 



