49 THE ORGAN OF HEARING. 



thickened at their nucleated basilar ends. The latter do not extend 

 to the basilar membrane but end at about the level of the center of 

 the inner pillars. At the free end of each cell is an elliptic cuti- 

 cular zone, somewhat broader than the end-surface of the corre- 

 sponding^ cell. In man about twenty rigid filaments, known as 

 auditory hairs, are found resting on each elliptic cuticular zone. 

 These are either arranged in a straight row or they describe a slight 

 curve. 



The middle division of the organ of Corti, the arches of Corti, 

 consists of long slender structures, known as pillar cells, or, briefly, 

 pillars, resting firmly upon the basilar membrane and forming an 

 arch at the vestibular side of the latter. They surround, by the 



x c d f 



^j-V\A ! \ ;" 



a b b K "V i 



i 



*r'G ( '- : ' ',.4 o . - ; t>,<-o, :r . q 



/ i^ ^ 



im 



i 



J 



\ 



Fig. 374. Organ of Corti : At x the tectorial membrane is raised ; <r, outer sus- 

 tentacular cells ; </, outer auditory cells ; f, outer pillar cells ; g, tectorial membrane ; h, 

 inner sustentacular cells; i, p, epithelium of the sulcus spiralis internus ; /&, labium ves- 

 tibulare ; e y tympanic investing layer ; m, outer auditory cells ; n, n, nerve-fibers which 

 extend through the tunnel of Corti ; o, inner pillar cell ; q, nerve-fibers ; b, b, basilar mem- 

 brane ; a, epithelium of the sulcus spiralis externus ; r, cells of Hensen ; j, inner audi- 

 tory cell ; /, ligamentum spirale (after Retzius). 



union of their free ends, a space which, as seen in figure 374, 

 appears triangular in section. This "is the tunnel of Corti. 



According to their position, we distinguish inner and outer 

 pillars, the inner being more numerous than the outer. Including 

 the entire extent of the lamina spiralis membranacea, we find that 

 there are about 6000 of the inner and 4500 of the outer pillar 

 cells. 



Each pillar cell originates from an epithelial cell, and is found 

 to be composed of a protoplasmic portion containing the nucleus, 

 which may be regarded as a remnant of the primitive cell, and of a 

 cuticular formation derived from the primitive cell, forming the 

 elongated body of the pillar cell the pillar. The free adjoining 

 ends are called the heads of the pillars. The head of the inner 

 pillar is provided with a flattened process, the head-plate, which 

 extends outward and forms an obtuse angle with the axis of the 

 pillar. Under this plate, and at the outer side of the head of the 



