THE INTERNAL EAR. 



925 



lying between the membrane of Reissner and the membrana tectoria, which con- 

 tains no object for description, while the space lying between the membrana tec- 

 toria and membrana basilaris is described by itself as a fourth canal the ductm 

 cochharis or duct us auditor/ us. 1 The latter is the space in which the organ of 

 C"/it' 2 is contained. This organ (Fig. 550) is situated upon the membrana basila- 

 ris, and appears at first sight as a papilla, winding spirally with the turns of this 

 membrane throughout the whole length of the cochlea, from which circumstance 

 it has been designated the papilla spiralis. More accurately viewed, it is seen +o 



FIG. 550. Floor of scala media, showing the organ of Corti, etc. 



be composed of a remarkable arrangement of cells which may be likened to the 

 kevboard of a pianoforte. Of these cells, the two central ones are rod-like 

 bodies, and are called the inner and outer rods of Corti. They are placed erect 

 on the basilar membrane at some little distance from each other, the space 

 between them being denominated the zona arcuata; they are inclined toward 

 each other, so as to meet at their opposite extremities and form a series of arches 

 roofing over the zona arcuata, thus forming a minute tunnel between them and 

 the basilar membrane, which ascends spirally through the whole length of the 

 cochlea. They are estimated at over three thousand in number. 



The inner rods, which are more numerous than the outer ones, rest on the 

 basilar membrane, close to the labium tympanic-urn: they project obliquely for- 

 ward and outward, and terminate above in expanded extremities, which resemble 

 in shape the upper end of the ulna, with its sigmoid cavity, coronoid and olecra- 

 non processes. On the outer side of the rod, in the angle formed between it and 

 the basilar membrane, is a protoplasmic cell, whilst on the inner side is a row of 

 epithelial cells surmounted by a brush of fine, stiff, hair-like processes, these cells 

 being continuous with the cubical cells lining the sulcus spiralis. 



The outer rods also rest by a broad foot on the basilar membrane : they 

 incline forward and inward, and their upper extremity resembles the head and 

 bill of a swan, the head fitting into the concavity the analogue of the sigmoid 

 cavity of one or more of the internal rods, and the bill resting against the 

 phalanges of the lamina reticularis. presently to be described. 



In the head of these outer rods is an oval portion, where the fibres of which 

 the rod appears to be composed are deficient, and which stains more deeply with 

 carmine than the rest of the rod. This is supposed to represent the nucleus of 

 the cell from which the rod was originally developed. At the base of the rod. on 

 its internal side that is to say, in the angle formed by the rod with the basilar 

 membrane is a similar protoplasmic cell to that found on the outer side of the 

 base of the inner rod, whilst external to the outer rod are three or four successive 



1 In reading the older descriptions of the organ of hearing the student must bear in mind that the 

 membranes bounding the ductus auditorius, together with the organ contained between them, were de- 

 scribed together as the " lamina spiralis membranacea. v while the membrane of Reissner was not recog- 

 nized, the parts being, in fact, as shown in the second turn of the cochlea on the right hand of Fig. 549. 



1 Corti's original paper is in the Zeitschrift f. Wissen. Zool., iii. 109. 



