THE EAR. 



361 



already described. The free surface of these cells is provided 

 with numerous hairs (Fig. 160, 3). 



The arrangement of the cells in the crista acustica is essen- 

 tially the same as that of the ma- 

 cula, with the exception that the 

 crista rests upon an infolding of 

 the wall called the septum nerveum 

 (Fig. 160, sep. nerv.), and has on 

 each side two half - moon - shaped 

 prominences of cylindrical epithe- 

 lium called the plana semilunata 

 (Fig. 160, pi. sem.), into which no 

 nerves have been traced. At the 

 point where the macula and plana 

 semilunata pass into the epithe- 

 lium lining the rest of the canal, 

 there is found an intermediate form 

 of cell, larger than the ordinary 

 epithelium, and separated one from 

 another by a fine web of connective 

 tissue. These have received the 

 name of protoplasmic cells, but as 

 yet their function has not been 

 discovered. 



Covering the crista in the place 

 of an otolith is a gelatinous mass 

 in the form of a cup, having a stri- 

 ated appearance, and into which 

 the fine hairs of the internal sur- 

 face project. This is considered as 

 a cuticular formation, and is sup- 

 posed to act as a damper (Fig. 160, 

 cupula). 



The nerve, after passing through 

 the wall at the point opposite the 

 crista or macula, loses all its 

 sheaths, and forms a fine plexus in 

 the outermost layer of cells, and 



this plexus has been found to communicate with the inner 

 filaments of the middle layer of cells, the internal filaments of 

 which ended as free cilia or were joined to cells of the inner 



FIG. 161. Separate cells from the ma- 

 cula, showing the connection of the cylin- 

 drical cells with the cells having thread- 

 like processes, and also the passage of these 

 processea to the surface between the cells. 

 Kuhn. 



