OF THE OENITHORHYNCHUS PLATYPUS. 
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(see Plate 46, fig. 8). This is called the tegmentum; it is a very vascular membrane, 
lined with large, rounded, granular, pigmented, epithelial cells, intermixed with some 
very transparent ones. 
Tracing the epithelial layer down to the triangular cartilage on one side, and the 
quadrilateral cartilage on the other, the cells change their character. Those lining 
the upper surface of the triangular cartilage and the adjacent portion of the membrana 
basilaris become transitional or cuboid in shape, with well marked nuclei. Those 
covering the upper surface of the quadrilateral cartilage are columnar, and of these 
there are two kinds. The contents of those nearer to the tegmentum are peculiar, 
resembling in appearance the matrix of ordinary cartilage, and this is readily coloured 
by staining agents. Further onwards, where the surface of the cartilage curves 
downwards to come to the level of the membrana basilaris, these cells are replaced by 
other columnar cells, the contents of which are very transparent. These are again 
succeeded by the mass of nerve epithelium, which may be termed the organ of Cortt. 
The organ of Corti is a broad band of nerve epithelium, resting partly on the upper 
surface of the quadrilateral cartilage and partly on the adjacent portion of the 
membrana basilaris. 
This organ of Corti consists chiefly of two layers of cells. One set is composed of 
nuclear cells (as described in the Mammalian organ of Corti) which lie on the 
membrana basilaris and surface of the quadrilateral cartilage. The other layer is 
situated at the surface of the organ ; the cells of this layer have distinct outlines, are 
rounded or elongated, have distinct nuclei, and they each bear on their summit a 
short thorn. These thorns are formed of several short bristles united together, and 
they project through holes in a membrane which limits the organ, and which is very 
similar to the membrana reticularis of the maculte acusticse and serving the same 
purpose. 
From this membrana reticularis . pass down numerous fibres or trabeculae to the 
membrana basilaris, binding the two membranes together. 
Covering the whole organ and also some of the columnar cells of the quadrilateral 
cartilage is a thick mucoid layer into which the thorns project; this mucoid layer is 
the membrana tectoria. 
The cochlear branch of the auditory nerve runs along the tube, first in the scala 
tympani and then in the substance of the quadrilateral cartilage. It gives off fibres 
all the way up, which immediately enter ganglionic cells, and then pass upwards 
towards that part of the upper surface of the cartilage on which the organ of Corti 
lies. 
Here the fibres pierce and enter the organ, not by a single row of perforations but 
by numerous apertures throughout the whole width of this portion of the quadrilateral 
cartilage. From these perforations nerve filaments pass in a radiating manner to the 
upper row of cells (hair cells), and possibly also to the lower cells, but their exact 
distribution has not been made out. 
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