1524 



HUMAN ANATOMY. 



FIG. 1277. 



Hind-brain 



Otic sac 



The semicircular canals differentiate from three folds which grow from the vestibular 

 pouch opposite the attachment of the ductus endolymphaticus. The central parts of the two 

 walls of each fold unite and undergo absorption, while the peripheral part of each fold remains 

 open, thus forming a semicircular tube, one end of which becomes enlarged to form the 

 ampulla. The superior vertical canal appears first, and the horizontal or external last. The 

 growth of the epithelial diverticula is later accompanied by a condensation of the surrounding 

 mesoblast, which differentiates into an external layer, the future cartilaginous and later bony 

 capsule ; a layer internal to this becomes the perichondrium and later periosteum. A second 

 mesoblastic layer is formed from the cells immediately surrounding the otic vesicle, whilst the 

 space between these fibrous layers is filled by a semi-gelatinous substance which later gives 

 place to the perilymph occupying the perilymphatic space. Within the ampullae, which early 

 develop, the epithelial lining undergoes specialization, accompanied by thickening of the meso- 

 blastic wall within circumscribed areas, to form the cristae acusticae. 



Coincidently with the development of the semicircular canals, a diverticulum, the cochlear 

 canal, appears at the lower anterior end of the membranous sac. This tube, oval in section, 



grows forward, downward, and inward, and represents 

 the future cochlear duct. After attaining considerable 

 length, further elongation is accompanied by coiling 

 and the assumption of the permanent disposition of 

 the tube. The epithelium of the cochlear tube early 

 exhibits a distinction, the cells of the upper surface 

 of the somewhat flattened canal becoming attenuated, 

 whilst those on the lower wall undergo thickening and 

 further differentiation. The flattened cells form the 

 epithelial covering of Reissner's membrane and of the 

 outer wall, and the taller elements are converted into the 

 complicated structures of the tympanic wall of the ductus 

 cochlearis, including the crista, the sulcus, and the 

 organ of Corti. 



The development of these structures includes the 

 differentiation of two epithelial ridges ; from the inner 

 and larger of these is derived the lining of the sulcus 

 spiralis and the overhanging membrana tectoria. The 

 outer ridge is made up of six rows of cells, the inner 

 row becoming the inner hair-cells, the outer three 

 rows becoming the outer hair-cells, whilst the two 

 rows between these two groups form the rods of 

 Corti. The crista appears between the sulcal cells and 

 the cochlear axis as a thickening of the spiral lamina. 



The cochlear outgrowth of the primary otic vesicle forms the membranous cochlea, or 

 scala media, alone, the walls of the adjacent divisions, the scala vestibuli and scala tympani, 

 resulting from the changes within the surrounding mesoblast. The latter differentiates into two 

 zones, an outer, which Incomes the cartilaginous, and finally osseous, capsule, and an inner, 

 lying immediately around the membranous canal, which for a time constitutes a stratum of deli- 

 cate connective tissue between the denser capsule and the ectoblastic canal. Within this layer 

 clefts appear, which gradually extend until two large spaces bound the membranous cochlea 

 above and below. 



Tlu-se spaces, the scala vestibuli and the scala tympani, are separated fora time from the 

 scala media by a robust septum consisting of a mesoblastic layer of considerable thickness and 

 the u.ill of tlie ectoblastic tube. With the further increase in the dimensions of the lymph- 

 paces, the partitions separating them from the cochlear duct are correspondingly reduced, 

 until, finally, the oner broad layers are represented by frail and attenuated structures, the 

 membrane of Reissner and the basilar membrane, which consequently include an ectoblastic 

 stratum, the epithelial layer, strengthened by a mesoblastic lamina, represented by the sub- 

 stantia propria and its endothelioid covering. 



The main s.ic of the ,,tic vesicle from which the foregoing diverticula arise constitutes the 

 primitive membranous vestibule, and later subdivides into the saccnle and utricle. This separa- 

 tion begins as an annular constriction of the primitive vestibule, incompletely dividing the vesicle 

 into tuo compartments. The still relatively large ductus endolymphaticus, the direct successor 

 "' ''' ' iid.'lvmphaticus, unites \\ith the narrow canal connecting these vesicles in such 



a 111. inner that each space receives one of a pair of converging limbs, an arrangement foreshad- 

 owing the permanent relations of the parts. 



K\en before the subdivision of the primitive vestibule is established, the vestibular end 

 of the cochlear canal beeomes eotistricted, so that communication between this tube and the 

 future saccule is maintained bv only a narrow passage, later the canalis reunions. The devel- 

 opment of the macula; acusticae of the saccule and utricle depends upon the specialization of 



Pan of frontal section of head of rabbit 

 embryo ; otic sac is separated from ectoblast 

 and beginning to elongate. X 40. 



