1880.] On the Cochlea of the Ornithorhynchus platypus. 151 



of the limbus, and is attached to the upper part of the ligamentum 

 cochlea?. 



The membrana basilaris is composed of three fibrous layers ; the 

 lower, longitudinal ; the middle, transverse ; and the upper, formed of 

 very fine transverse fibres. There are two blood-vessels running longi- 

 tudinally in the lower layer. 



The ligamentum cochleae is a somewhat triangular mass of con- 

 nective tissue with numerous blood-vessels, which in its upper portion 

 run longitudinally, and, with the epithelium, form the stria vascularis. 



The membrane of Reissner is composed of a delicate basement 

 membrane, with the endothelium of the scala vestibuli on its upper 

 surface, and epithelium on its under surface ; here and there blood- 

 vessels may be traced on it, running from the limbus to the ligament, 

 and in some places these vessels form convoluted knots. 



The epithelium lining the ductus cochleae varies according to its 

 position: that lining the membrane of Reissner is composed of a 

 single layer of hexagonal cells ; in the sulcus they are rounded ; on the 

 inner part of the membrana basilaris and the lower portion of the 

 ligament they are cuboid ; on the upper part of the ligament they are 

 very peculiar, and resemble the transitional variety closely packed 

 together. In the deeper part of the layer run numerous longitudinal 

 blood-vessels, and this forms the already mentioned stria vascularis. 

 The remaining portion of the epithelial layer that lies on the lower lip 

 of the sulcus, and on the outer portion of the membrana basilaris, is 

 developed into the so-called organ of Corti. 



This organ of Corti consists of a double row of rods (of Corti), 

 united at their upper ends and separate below; they stand on the 

 membrana basilaris, and with it form a triangular tunnel. 



The rods of both rows have cylindrical shafts and enlarged ex- 

 tremities ; the upper ends of the inner row are rounded, and fit in 

 corresponding concavities of the outer row. A delicate process pro- 

 jects inwards from the upper part of each of the rods, the processes of 

 the outer ones lying above those of the inner. 



Rows of hair cells are arranged on either side of these rods — one* 

 to the outer side and three to the inner. Below the three inner rows 

 are situated rows of nuclear cells (cells with well-marked nuclei, but 

 no regular cell-wall), the cells of Deiters. Lying on the lower lip of 

 the sulcus is a small mass of nuclear cells, and there is a row of these 

 cells at each of the lower angles of the triangular tunnel. 



The inner and outer side of the organ of Corti is formed of modified 

 columnar cells. A reticulate membrane covers the rods and hair cells, 

 the hairs of which project through certain circular meshes of the 

 membrane. 



* Since presenting this communication I have discovered a second row of hair 

 cells in this position. 



