230 DISSECTION OF THE DOG 



osseus), containing membranous sacs and tubes — the membranous labyrinth 

 (labyrinthus membranaceus). The central space of the osseous labyrinth, the 

 vestibule (vestibulum), in the dried skull communicates with the cavity of the 

 tympanum by an oval opening, the fenestra of the vestibule (fenestra vestibuli). 

 In the fresh state and in the living animal the fenestra is occluded by the base 

 of the stapes, one of the auditory ossicles. In the posterior part of the vesti- 

 bule are fine openings leading into the three semicircular canals (canales semi- 

 firculares ossei), while in the lower and anterior part of the cavity is an opening 

 into the spiral canal of the cochlea. The cochlea itself resembles the shell of 

 a snail, and consists essentially of the spiral canal wound round a central 

 column of bone designated the modiolus. The spiral canal ends blindly at the 

 apex of the cochlea, where it forms the cupola (cupula). Into the canal a 

 spiral lamina of bone (lamina spiralis ossea) projects for some distance from 

 its inner (modiolar) wall ; thus the canal is partially divided into two com- 

 partments ; one of which, the scala vestibuli, communicates with the vestibule, 

 the other, the scala tympani, in the macerated bone opening by a small rounded 

 aperture — the fenestra of the cochlea (fenestra cochlea?) — into the cavity of 

 the tympanum. In the fresh condition the fenestra is closed by a membrane. 



The membranous labyrinth consists of the saccule, the utricle, three 

 semicircular ducts and the cochlear duct. The saccule (sacculus) and the 

 utricle (utriculus) are lodged in the vestibule, and are placed in indirect 

 communication with each other by means of the endolymphatic duct (ductus 

 endolymphaticus). The semicircular ducts (ductus semicirculares) occupy the 

 three bony semicircular canals and are in communication with the utricle. 



The cochlear duct (ductus cochlearis) is attached, on the one side to the 

 outer wall of the spiral canal of the cochlea, and on the other to the edge of 

 the bony spiral lamina. At the base of the cochlea the duct is connected with 

 the saccule by a narrow uniting duct (ductus reuniens). The blind apical 

 termination of the duct does not quite reach the cupola of the cochlea, thus 

 allowing a communication between the scala vestibuli and the scala tympani. 



Dissection. — As previously stated, it is scarcely possible to make a com- 

 plete dissection of the ear on one specimen only. In order to examine 

 the cavity of the tympanum the lower part of the osseous bulla should 

 be removed bit by bit, a ring of bone being left round the external 

 acoustic meatus. In order to demonstrate the tympanic membrane 

 properly, it is necessary to remove the cartilages of the auricle. It 

 will be observed that the cavities of the tympanum and of the osseous 

 bulla communicate with each other by a wide opening bounded by the 

 serrated edge of a thin plate of bone. 



Cavum tympani. — The tympanic cavity is a narrow, air-containing 

 chamber between the internal ear and the end of the external acoustic meatus. 

 Its outer wall is formed almost entirely by the thin, translucent tympanic 



