NORMAL HISTOLOGY. 



After ascending about half-way to the free surface the fibres break up 

 into their fibrillae, many of which are distributed to the hair-cells, 

 with which they probably stand in close relation, while others pass 

 as free axis-cylinders between the epithelial elements to a higher 

 level. 



The blood-vessels of the saccule and the utricle form a wide- 

 meshed capillary net-work within the fibrous wall of the membranous 

 sacs, the vascular supply being especially rich within the maculae 

 acusticse. 



THE SEMICIRCULAR CANALS. 



The inner surface of the bony capsule constituting this part of 

 the osseous labyrinth is lined by a thin periosteum similar to that 

 of the vestibule already described. Along the line of attachment of 



the membranous canal this layer sends 

 FIG. 398. o ff numerous connecting bundles of 



fibrous tissue ; in other parts of the 

 circumference of the canal only widely- 

 separated , occasional trabeculae 

 bridge across the perilymphatic space 

 to aid in maintaining the position of 

 the membranous tube. The inner 

 surface of the periosteum, the tra- 

 beculae, and the outer face of the 

 fibrous tunic of the membranous 

 canals are invested by the endothe- 

 lium which forms the immediate 

 lining of the perilymphatic space. 

 The walls of the membranous 

 semicircular canals closely re- 

 semble those of the saccule and the 

 utricle, being made up of an outer 

 fibrous lamella and an inner epi- 

 thelial lining. The fibrous coat 

 is further differentiated into an external layer of felted connective- 

 tissue bundles, containing many cells, and an inner, more compact, 

 almost homogeneous layer, which corresponds to a highly-developed 

 basement-membrane. 



The epithelium of the semicircular canals, supported by the outer 

 fibrous coat, consists throughout the greater part of its extent of a 

 single layer of flattened polyhedral cells (12-18 />.) similar to 

 those lining the saccule and the utricle. 



The areas receiving the terminal filaments of the auditory nerve, 

 the cristse acusticse, are distinguished by specialization of the 



Section of wall of cat's semicircular 

 canal : a, epithelial lining of canal ; b, 

 basement-membrane ; c, fibrous tunic 

 united with osseous lamella (_/) by tra- 

 beculae (d, d) ; e, blood-vessel. 



