:;7i' 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



external or horizontal semicircular canal. Their planes are at right angles to 

 one another, so that they occupy the three possible dimensions of space. The 

 externa] canal lies in a nearly horizontal plane, while the other two approach 

 the vertical. Each canal is dilated at one extremity into a globular cavity 



which is more than twice the diameter of the 

 canal itself, and which is known as the am- 

 putta. The anterior and posterior canals 

 unite near the ends not provided with am- 

 pulla 1 , and they enter the vestibule as a com- 

 mon tube. Anteriorly the cavitv of the 



— 4 vestibule is continued as a tube of complex 

 internal structure which is coiled upon itself 



l'i'.. 1'."'.- I tiiiLT.-iin lift he osseous cochlea _ _ l 



laid open (after Quain) : I, scala vestibuli ; two and one-half times, and which, from its 



LMamina spiralis. 3 scala tympani ; 4, cen- regemb l ance to the ghell of a snai ] j g kncnyn 

 tral pillar or modiolus. ' 



as the cochlea (PI. 1, Fig. 3). The osseous 

 cochlea may be conceived as formed by a bony tube turned about a bony central 

 pillar, the modiolus, which diminishes in diameter from the base to the apex 

 of the cochlea. From the modiolus a bony shelf stretches into the cavity of 

 the tube, incompletely dividing it into two tubular chambers, winding round 

 the modiolus like a circular staircase, the upper of which chambers we shall 



—9 



Fig. 191— Diagram of right membranous labyrinth seen from the external side (after Testut) : 1, utri- 

 cle ; 2, 3, 4, superior, posterior, and horizontal semicircular canals; 5, saccule: 6, ductus endolymphat- 

 lcus, with 7, 7', its twigs of origin : 8, Baccus endolymphaticus : 9, canalis cochlearis, with 9', its vestibular 

 cul-de-sac, and 9", its blind extremity ; 10, canalis reunions. 



soon learn to know as the scala vestibuU, and the lower chamber as the scala 

 tympani ( Fig. 1 J»0; PI. 1, Fig. 3). The bony shelf mentioned above as partly 

 bisecting the cochlear tube has, of course, like the latter, a spiral course, and is 

 known as the lamina spiralis; its importance as a supporter of the auditory- 

 nerve filaments will soon be seen. 



Contained within the cavitv of the bony labyrinth, and parallel with its walls, 

 is the membranous labyrinth, in which are found the essential structures of the 

 organ of hearing (PI. 1, Fig. 4 ; Fig. 11*1 ). The membranous labyrinth is filled 

 with a somewhat watery, mucin-holding fluid, the endolymph, while a similar 

 fluid, the perilymph, is found outside it and within the osseous labyrinth. The 



