496 THE ORGAN OF HEARING. 



under the dura in a saccus endolympliaticus. In connection with 

 the latter are epithelial tubules bordering upon lymph-channels, 

 with which they probably communicate by means of interepithelial 

 (intercellular) spaces (Riidinger, 88). The efferent channels for 

 the perilymph of the vestibule extend along the nerve sheaths of 

 those nerves supplying the maculae and cristae ; these passageways 

 finally communicate with the subdural or subarachnoid spaces. 

 The perilymph of the cochlea is carried off by the adventitious 

 tissue of the vena aqueductus cochleae, the lymph-vessels of which 

 empty into certain subperiosteal lymph-channels near the inner 

 margin of the jugular fossa. 



4. ON THE DEVELOPMENT OF THE LABYRINTH. 



In man the epithelium lining the membranous labyrinth origi- 

 nates from the ectoderm as a single-layered epithelial vesicle, the 

 auditory vesicle or the otocyst, during the fourth week of embryonic 

 life. After being constricted off from the ectoderm, this vesicle 

 lies in the vicinity of the epencephalon and is surrounded by mesen- 

 chyme. The auditory vesicle then develops a dorsomesial evagina- 

 tion, which gradually grows larger and finally becomes the ductus 

 endolympliaticus. An evagination also occurs in the ventral wall 

 of the vesicle, the recessus cochlea. At the same time the mesial 

 wall is pushed inward, thus incompletely dividing the vesicle into 

 two smaller sacs the dorsal utriculus and the ventral sacculus. 

 From the utricular portion there arises a horizontal evagination, 

 flat and quite broad the first trace of the lateral or horizontal 

 semicircular canal ; soon after, another evagination, vertical and 

 still broader than the first, is seen the anlage of the other two 

 canals. The outer portion of these pouches gradually expands, 

 while in the middle, the two layers of each evagination come in 

 contact with each other and coalesce, finally becoming absorbed. 

 In the vertical evagination two such areas of adherence are found, 

 thus forming a superior and a posterior canal, both having a com- 

 mon crus at one end. 



The recessus cochleae grows both in a longitudinal and in a spiral 

 direction, forming the cochlear duct. 



In the immediate vicinity of the membranous labyrinth, the 

 mesenchyme is differentiated into a connective -tissue wall for the 

 former. The successive layers of mesenchyme, except in those 

 areas where the membranous labyrinth later becomes adherent to 

 the osseous, are transformed into a mucous connective tissue. The 

 latter is surrounded by a more compact tissue, from which are de- 

 rived, first, cartilage; then bone and periosteum, and thus, finally, 

 the osseous labyrinth. By a peculiar process of regressive meta- 

 morphosis most of the mucous connective tissue later disappears. 

 In the adult it is replaced by the perilymphatic spaces of the laby- 

 rinth. 



