370 HUMAN ANATOMY. 



surface of the membrana basilaris, and forming a series of 

 arches. Between the bases of the arches is the zona arcuata. 

 There are over three thousand arches, and within them is formed 

 a tunnel extending the entire length of the lamina spiralis termi- 

 nating at the hamular process. 



Inner rods of Corti, rest upon the basilar membrane. Their 

 upper extremities resemble the proximal extremity of the ulna, 

 and each has on its inner side a row of epithelial cells the 

 inner hair-cells. 



Outer rods of Corti, are attached in a similar manner at 

 the bases, and their extremities each resemble the head and bill 

 of a swan, the former fitting into the cavity in the inner rods. 

 On the outer side are four rows of ciliated cells the outer hair- 

 cells. 



Reticular membrane,, or lamina reticularis, is a delicate 

 complex net-work of phalanges fiddle-shaped structures ex- 

 tending from the inner rods to the external row of the outer 

 hair-cells. 



THE INTERNAL AUDITORY CANAL, is a short canal about one- 

 third of an inch in length, extending outward from the meatus 

 auditorius internus, on the posterior surface of the petrous por- 

 tion of the temporal bone to terminate at a vertical, perforated 

 plate the lamina cribrosa. This plate is divided by a horizontal 

 ridge into a lower and upper portion. The lower is perforated 

 by numerous foramina for passage of the auditory artery and 

 branches of the auditory nerve, some of which are arranged into 

 a spiral-shaped depression, called the tractus spiralis foraminu- 

 lentus; the upper has one large opening the commencement 

 of the aquseductus Fallopii for portio dura, or seventh nerve. 



Auditory Nerve. The auditory nerve enters the internal 

 auditory meatus in company with the facial, and at the bottom 

 divides into two branches the vestibular and cochlear. 



Vestibular nerve subdivides into three branches, as fol- 

 lows : 



Superior, distributed to the utricle and ampullae of the superior 

 and external semicircular canals : 



Middle, enters the bottom of the fovea hemispherica, and is dis- 

 tributed to the saccule ; and the 



Inferior, is distributed to the ampulla of the posterior semicircular 

 canal. 



Cochlear nerve,, ascends in the canals in the modiolus, and 

 divides into numerous branches which pass between the plates 

 of the lamina spiralis ossea, forming a plexus which contains 

 the ganglia spirale, from which filaments are distributed to the 

 outer and inner hair-cells of the organ of Corti. 



