THE SENSE OF HEARING. 



641 



FIG. 313. BONY COCHLEA, i. 

 Ampulla of superior semi- 

 circular canal. 2. Horizontal 

 canal. 3. Junction of supe- 

 rior and posterior semicircu- 

 lar canals. 4. The posterior 

 semicircular canal. 5. Fora- 

 men rotundum. 6. Foramen 

 ovale. 7. Cochlea. 



scala is in free communication with the vestibule, and is known as 

 the scala vestibuli; the lower passage or scala in the dead condition 

 communicates with the tympanum by means of a round opening, 

 the foramen rotundum, and is therefore known as the scala tympani. 

 In the living condition this opening is completely closed by a mem- 

 brane, a second membrana tympani. 

 Both the scalae vestibuli and tympani 

 communicate at the apex of the 

 cochlea by a small opening, the heli- 

 cotrema. The modiolus, the central 

 bony axis, is perforated from base to 

 apex by a canal for the passage of the 

 auditory nerve-fibers; lateral canals, 

 diverging from the central canal, pass 

 through the osseous lamina spiralis 

 and transmit fibers of the auditory 

 nerve. The interior of the bony laby- 

 rinth is lined by periosteum covered by 

 epithelium and in communication with 

 lymph-spaces at the base of the skull by 

 means of the aqueduct of the vestibule. 

 The membranous labyrinth, 

 lying within the osseous labyrinth, corresponds with it in form, though 

 it is smaller in size. It may be subdivided into vestibule, semi- 

 circular canals, and cochlea (Fig. 314). 



The vestibular portion consists of two small sacs, the utricle and 

 the saccule, which communicate with each other by means of the 



two branches of a duct passing through 

 the aqueduct of the vestibule the ductus 

 endolymphaticus . 



The semicircular canals communicate 

 with the utricle in the same manner as 

 the bony canals communicate with the 

 vestibule. The saccule communicates 

 with the membranous cochlea by a short 

 canal, the canalis reuniens. The walls of 

 the utricle, saccule, and semicircular 

 canals are composed of connective tissue 

 lined by epithelium. At the points of 

 entrance of the auditory nerve, the 

 macula acustica, in all three structures, the epithelium undergoes a 

 marked change in appearance and structure. It becomes columnar 

 in shape and provided with stiff hair-like processes or threads, 

 which project into the cavity. In the saccule and utricle the hair- 

 like processes are covered by a layer of small crystals of calcium 



FIG. ,314. i. Utricle. 2. Sac- 

 cule. 3. Vestibular end of 

 cochlea. 4. Canalis reuniens. 



5. Membranous cochlea. 



6. Membranous semicir- 

 cular canal. (Potter's 

 "Anatomy.") 



