DEVELOPMENT OF THE EAR. 



becomes mucoid in appearance and cavities lined with mesenchymal 

 epithelium are formed within it. They contain a tissue fluid called peri- 

 lymph. Around the semicircular ducts the perilymph spaces are so large 

 that the tissue between them is reduced to strands as shown in Fig. 431; 

 these are sometimes called ligaments. The perilymph spaces around the 

 semicircular ducts are irregularly arranged and communicate with one 

 another at various points, but those around the cochlear duct form a single 

 tube. It arises from the other spaces at the base of the cochlea and covers 

 the lateral or outer surface of the ductus cochleae as it ascends to the 



Semicircular duct. 



/ * . / 



,,';< "\ ;<* ','v^' / < : S5s,V. . Epithelium of the 



F; ; ^:V.v3>V-'^1*j?~~ duct- 



Blood vessel. 



Wall of the semi- 

 circular duct. 



Ligament of the 

 duct. 



Bone of the semi- 

 circular canal. 



Ligament.' 



Perilymph spaces. 



Blood vessel. 



FIG. 431. CROSS SECTION OF A SEMICIRCULAR DUCT AND THE ADJACENT PERILYMPH SPACES TO- 

 GETHER WITH THE SEMICIRCULAR CANAL OP BONE IN WHICH THEY ARE LODGED. From a human 

 adult. X so. (Bohm and von Davidoff.) 



cupula; there it turns and descends along the medial or inner surface of the 

 ductus cochleae, ending blindly at the base not far from its origin. The 

 ascending perilymph space excavated in the mesenchyma around the coch- 

 lear duct is the scala vestibuli. The descending space with which it con- 

 nects at the cupula is the scala tympani. The arrangement of the cochlear 

 duct and its scalae is shown in the section through the axis of the spiral, 

 Fig. 432. The upper side of the figure is directed forward and outward 

 in relation to the body. 



The temporal bone develops from the mesenchyma surrounding the 



