820 



DISSECTION OF THE EAR. 



The mem- 

 branous 

 labyrinth 

 consists of 

 utricle, 

 saccule, 

 semicircular 

 canals, and 

 cochlear 

 canal. 



Utricle : 



situation, 



and form ; 



macula, 



andotoliths. 



Semicircular 

 canals : 

 not free in 

 cavity ; 



have 

 ampullae, 



which 

 receive the 

 nerves. 



MEMBRANOUS LABYRINTH (fig. 305). Lodged in the vestibule 

 are two membranous sacs, the utricle and saccule from the former; 

 of which tubular offsets are continued into the semicircular canals. 

 These, together with the canal of the cochlea and the organ of Corti, 

 which have been referred to above, make up the membranous 

 labyrinth. The sacs and their prolongations are immersed in the 

 perilymph, and are themselves filled with a fluid called the 

 endolymph. In them the ramifications of the auditory nerve are 

 distributed. 



Dissection. The delicate internal sacs of the ear, with their 



nerves, cannot be dissected 

 except on a temporal bone 

 which has been softened in 

 acid, and afterwards put in 

 spirit. The previous instruc- 

 tions for the dissection of the 

 osseous labyrinth will guide 

 the student to the situation 

 of the membranous structures 

 within it, but the surrounding 

 softened material must be re- 

 moved with great care. 



The UTRICLE (fig. 305, rf), 

 or the common sinus, is the 

 larger of the two sacs, and is 

 situate at the posterior and 

 upper part of the vestibule, 

 opposite the fovea hemiellip- 

 tica in the roof. It is trans- 

 versely oval in form, and con- 

 nected with it posteriorly are 

 three looped tubes, which 

 occupy the semicircular canals. 

 At the fore part of the sac is 

 a thickened and more opaque 

 part of its wall macula acus- 

 tica (e), where the nerves 

 enter ; and opposite this, in 



FIG. 305. PETROUS BONE PARTLY RE- 

 MOVED TO SHOW THE MEMBRANOUS 



LABYRINTH OF THE LEFT SIDE IN 

 PLACE (BRESCHET). 



a. Saccule. 



b. Its macula. 



c. Ductus reuniens. 



d. Utricle. 



e. Its macula. 



f. Ampullary enlargement of the ex- 



ternal semicircular canal, g. the interior, is a small mass of 



calcareous granules or otoliths. 



The MEMBRANOUS SEMICIRCULAR CANALS (g) are about one-third 

 of the diameter of the osseous tubes, along the convex border of 

 which they lie, being closely attached to the periosteal lining of the 

 bony wall ; and the remaining space is filled by perilymph. Each 

 is marked at one end by an ampulla, which is relatively of large 

 size and nearly fills the osseous case. Two are blended at one end, 

 like the canals they occupy, so that they communicate with the 

 utricle by five openings. At each ampullary enlargement there is 

 a transverse projection (crista acustica) into the anterior of the tube ; 

 and at that spot a branch of the auditory nerve enters the wall. 



