1555 A MANUAL OF ANATOMY 



tympani. The scala media terminates above in a closed extremity 

 at the cupola, and it has a similar mode of termination at the base 

 of the cochlea. Near its lower blind extremity it receives the 

 canalis or ductus reuniens of Hensen, by which it communicates 

 with the saccule. 



As just stated, part of the floor of the scala media is formed by 

 that portion of the upper surface of the lamina spiralis which lies 

 external to the attachment of the membrane of Reissner. In the 

 recent state it is of some thickness, which is due to a thickening 

 of its periosteal covering. This fibrous thickening forms the 

 limbus laminae spiralis. Its outer margin is crescentic, the deep 

 notch being called the sulcus lamincB spiralis. The sulcus has two 

 lips, upper and lower. The upper lip is called the labium, vesti- 

 bular e, the. upper surface of which is marked by several interlacing 

 prominences and grooves. At the free margin of this labium the 

 prominences assume the form of tooth-like projections, which are 

 known as the auditory teeth. The lower lip of the sulcus is called 

 the labium tympanicum. It is continued into the basilar membrane, 

 and is perforated by a great number of apertures for the branches^ 

 of the cochlear division of the auditory nerve. 



Organ of Corti. — Over the upper surface of the inner part (zona 

 arcuata) of the basilar membrane the epithelium undergoes remark- 

 able modification, and gives rise to the organ of Corti. The con- 

 stituent parts of this very complicated organ are as follows : 



1. The rods of Corti. 4. The cells of Hensen and of Claudius. 



2. The auditory or hair-cells. 5. The lamina reticularis. 



3. The cells of Deiters. 6. The membrana tectoria. 



The rods of Corti are arranged in two rows, inner and outer. The 

 rods of the inner row are spoken of as the inner rods, and those of 

 the outer row as the outer rods. Each rod consists of a foot-plate 

 or base, an intermediate portion, and a head, which latter represents 

 the upper extremity. The foot-plate, which is expanded, rests upon 

 the zona arcuata of the basilar membrane, and the foot-plates of 

 the inner rods are separated from those of the outer rods by a slight 

 interval. As the rods rise the intermediate portions of the inner 

 and outer rods incline towards each other, and the heads of the two 

 sets of rods come into contact. In this manner a triangular tunnel 

 is enclosed between the two sets of rods and the basilar membrane, 

 which is called the tunnel of Corti. This tunnel extends along 

 the entire length of the scala media, or ductus cochlearis. 



The inner and outer rods present certain diiferences. 



The inner rods are more numerous than the outer, there 

 being from 5,000 to 6,000 of the former, and about 4,000 of the 

 latter. They incline upwards and outwards. The head of each is 

 directed outwards and presents a concavity, above and below 

 which there is a projecting portion, so that it resembles, somewhat, 

 the upper extremity of the ulna, with its great sigmoid cavity and 

 olecranon and coronoid processes. The concavity on the head of 



