ORGAN OF HEARING. 



537 



osseous labyrinth is contained an extremely 

 delicate and complicated membrane - ner- 

 vous apparatus, called, membraneous laby- 

 rinth, first properly described by Scarpa.* 

 It does not extend into all the compartments 

 of the osseous labyrinth, but only occupies the 

 vestibule and semicircular canals. The coch- 

 lea, as has been said, contains in its cavity 

 nothing but perilymph. 



The vestibular part of the membraneous 

 labyrinth, and of that perhaps one of the 

 pouches only, is all that is really fundamental 

 in the structure of an organ of hearing. In 

 the Crustacea and Cephalopodous Mollusca in 

 which the organ of hearing exists in its sim- 

 plest form, and even in the Cyclostomatous 

 fishes there is nothing but a small pouch con- 

 taining a little liquid and a lapilliform body. 



Much smaller than the cavities which con- 

 tain it, the membraneous labyrinth is sus- 

 pended as it were in the perilymph. It does 

 not appear to adhere to the walls of the laby- 

 rinthic cavity except at the points where it re- 

 ceives nervous filaments. 



The component parts of the membraneous 

 labyrinth are: 



1. The common sinus. 2. The membrane- 

 ous ampullae and semicircular tubes. 3. The 

 saccule. 



Fig. 238. 



A magnified representation of the left osseous laby- 

 rinth laid open to show the membraneous labyrinth 

 in its situation. (From Breschet,) 



a. membraneous ampulla of the ampullary sinus 

 of the anterior semicircular canal ; b. membrane- 

 ous ampulla of the ampullary sinus of the external 

 semicircular canal ; c. membraneous ampulla of 

 the ampullary sinus of the posterior semicircular 

 canal; d. anterior membraneous semicircular tube; 

 e. external membraneous semicircular tube ; f. 

 posterior membraneous semicircular tube ; g. com- 



* De auditu et olfactu. 



mon membraneous tube resulting from the junction 

 of the tubes d and f; h. the place where the ex- 

 ternal membraneous semicircular tube opens into 

 the common sinus ; i t. common sinus filling a 

 great part of the vestibule ; k. a small mass of 

 calcareous powder shining through its walls ; / /. 

 saccule, also containing, m. another mass of cal- 

 careous powder ; n a nervous fasciculus, furnishing, 

 o. an expansion to the anterior membraneous am- 

 pulla ; p. another to the ampulla of the external 

 tube, and q. a third to the common sinus ; r. ner- 

 vous fasciculus to the saccule ; another fasciculus 

 of nervous filaments, not lettered, is seen going to 

 the ampulla of the posterior membraneous semi- 

 circular tube ; s s. spiral lamina ; '. the termin- 

 ation of the spiral lamina in the hamulus ; /. com- 

 mencement of the scala tympani near the fenestra 

 rotunda, which is here no longer seen ; . com- 

 mencement of the scala vestibuli ; x. extremity of 

 the axis around which the termination of the spiral 

 lamina turns ; y y. a bristle engaged in the heli- 

 cotrema ; z. place where the summit of the axis 

 is continued into the wall of the osseous labyrinth ; 

 www, membraneous portion of the spiral lamina, 

 particularly broad in the last turn, (lettered u 

 in the figure instead of w w w~) ; ****** spaces 

 between the walls of the labyrinthic cavity and 

 membraneous labyrinth occupied by the peri- 

 lymph. 



Fig. 239. 



The left membraneous labyrinth isolated together with 

 the nerves. Magnified. ( From Breschet.) 



a. ampulla of the anterior semicircular tube ; 



b. ampulla of the horizontal semicircular tube ; 



c. ampulla of the posterior semicircular tube ; d. 

 common tube ; e. mass of calcareous particles 

 lying in the common sinus ; /. the saccule con- 

 taining also a mass of calcareous particles; k. 

 portio dura of the seventh pair ; m. nervous fila- 

 ments to the ampulla of the anterior semicircular 

 tube ; n. filaments to the ampulla of the hori- 

 zontal semicircular tube; filaments are also seen 

 going to the ampulla of the posterior semicircular 

 tube, not lettered ; o. filaments to the common 

 sinus ; q. filaments to the saccule ; r. cochlear 

 nerve. 



The common sinus, membraneous ampulla, 

 and membraneous semicircular tubes. - These 

 constitute but one apparatus which is just the 

 counterpart of the vestibule, ampullary sinuses, 

 and semicircular canals of the osseous laby- 

 rinth ; the semicircular tubes opening into the 

 ampullae and common sinus in the same way 



