THE EAR. 359 



tions of the canal where the nerve terminates, and the simi- 

 lar portion of the saccule and utricle, are to be placed for 

 twenty-four hours in a saturated solution of gum arable in 

 water, and then directly into strong alcohol for twenty-four 

 hours longer, when they will be ready for embedding. The 

 sections, made with a sharp razor, kept well wet with alcohol, 

 are to be deprived of their gum by passing a stream of distilled 

 water beneath the cover-glass, the water being replaced by a 

 solution composed of one part of a saturated solution of ace- 

 tate of potash and four parts each of glycerine and water. 



The structure and arrangement of the semicircular canals, 

 except at the points of expansion of the nerve, is as follows : 

 In the osseous fishes the canals lie embedded in a mass of adi- 

 pose tissue, and are held in place by very short bony tubes ; 

 in the cartilaginous fishes (shark, skate) they lie in canals hol- 

 lowed out in the cartilage, while in man and the higher verte- 

 brates they are surrounded by bony walls. 



In man the membranous part does not entirely fill up the 

 bony canals, but is adherent to the lining periosteum at one 

 point, and to the rest of the wall by bands of connective tissue 

 (called ligamentum labyrinthi canaliculorum et sacculorum), in 

 the interstices of which the perilymph circulates. In the fishes 

 the walls of the tubes and ampullae, as well as of the utricle 

 and saccule, are composed of what has been termed spindle- 

 cartilage. This consists of a homogeneous ground-substance, 

 like that of ordinary cartilage, in which lie embedded long, 

 spindle-shaped connective-tissue corpuscles, anastomosing with 

 each other in all directions, like the corpuscles of the cornea. 

 The whole is lined with a pavement-epithelium. In man the 

 structure is different. Here there are to be distinguished three 

 layers, viz., externally, a layer of connective tissue, composed 

 of fibrous tissue with numerous nuclei. This is connected at 

 one point with the periosteum, and passes into the ligamenta 

 labyrinthi canaliculorum at the other points of the circumfer- 

 ence ; secondly, of a hyaline layer, the tunica propria ; this is 

 raised into papilliform projections in certain parts of the tube. 

 The internal layer is composed of simple pavement-epithelium. 



The distribution and termination of the nerve is as follows 

 in the fishes : The acoustic nerve divides into two branches, 

 the cochlear and vestibular, each of which gives off three fila- 

 ments. Those from the cochlear portion supply the saccule, 



