€56 THE EAR. 



ported by any plate of bone. The exact nature of the tissue composiHj^ 

 the proper substance of the membrane is unknown, but it is probably 

 analogous, at least the uppermost stratum, to that composing the rods 

 of Corti, to be afterwards described. It is somewhat stiff in con- 

 sistence, and maj readily be broken up into straight fibres which have 

 a radial direction, corresponding with a striation which the membrane, 

 especially its outer part, presents when view-ed on the flat (fig. 470). 

 Externally, at its attachment to the spiral ligament, it breaks up 

 into diverging fibres, which spread into that projection. On the upper 

 surface of the membrane is the epitheliumi which forms the organ of 

 Corti, and the single layer of cells which is continued from this ex- 

 ternally (fig. 469) : on the under surface it is covered by a layer of 

 connective tissue (often described as part of the membrane), the fibres 

 of which have a longitudinal direction, parallel with the spiral, and across 

 the direction of the fibres of the membrane proper. There are numerous 

 intermixed spindle-shaped corpuscles in this tissue, which is in con- 

 tinuity with the lining periosteum of the scala tympani. Small blood- 

 vessels are found in it, but as a rule extending only over the inner part 

 of the membrane. They are usually terminated by a rather larger 

 longitudinally running vessel, situate opposite the outer rods of Corti, 

 and known as the ras apiraU. 



The membrane of Reissner (figs. 467, 469, B), separates the scala 

 vestibuli from the canal of the cochlea. It is composed of an exces- 

 sively delicate and almost homogeneous layer of connective tissue con- 

 tinuous with the lining periosteum of the scala vestibuli, and is covered 

 on the surface which is turned to the cochlear canal with a simple 

 layer of delicate scaly epithelium continuous below with that on the 

 limbus and above Avith that lining the outer wall of the canal (fig. 469). 

 The cells have each a circular flattened nucleus, and not unlrequently 

 contain fat droplets of various sizes. The vestibular side of the mem- 

 brane of Eeissner is quite smooth, and is covered with a layer of 

 flattened epithelioid connective-tissue cells, readily distinguishable from 

 the true epithelial cells on the other side as well by their greater delicacy 

 of outline, and their larger size, as by the history of their development. 

 A few blood-capillaries are continued into the membrane from the 

 neighbouring periosteum. 



The periosteum which lines the scala vestibuli and scala tympani, 

 consists of ordinary connective tissue. There is no continuous lining 

 of flattened cells on the free surface (that bathed by the perilymph), 

 such as covers the surface of serous membranes. That, on the other 

 hand, which bounds the canal of the cochlea externally, is very much 

 thickened by a development of retiform connective tissue, and is 

 covered by the epithelium of that tube, which here forms a single layer 

 of cubical cells, many of which contain pigment. The periosteum, 

 moreover, forms usually a slight inward projection a little above the 

 ligaraentum spirale, containing a prominent blood-vessel (fig. 469, 

 L. sj). a). The substance of the periosteum is also fi'equently pigmented 

 immediately under the epithelium of the tract between this prominence 

 and the membrane of Reissner, and from containing large and nume- 

 rous blood-vessels, is often termed stria vascidaris {St.v.). Immediately 

 beneath the epithelium is a basement membrane, through which, in 

 sections, processes may here and there be seen passing from the 

 epithelium to the subjacent tissue. 



