78 AYERS. [Vol. VI. 



into the lamina spiralis ossea and supporting bone. The coch- 

 lear canal is triangular in section throughout most of its course, 

 and has the same general position within the lymphatic space 

 formed by the two scalae — s. tympani and vestibuli — that it 

 has in the higher mammals. The basilar membrane is a three- 

 layered, relatively thick plate. The lower layer is composed of 

 longitudinal fibres with spindle-shaped nuclei ; the middle layer 

 contains transverse fibres, while the upper layer is composed of 

 fine transverse fibres. 



The pillars of Corti are well developed, and they inclose a 

 triangular tunnel much as in mammals generally. The pillar 

 feet of the inner and outer pillars are very similar in shape and 

 size, but the enlarged heads diflfer ; that of the inner rod is 

 oblong in shape with a delicate process projecting inwards from 

 its upper part, while that of the outer pillar is rounded and fits 

 into a hollow of the head of the inner rod. As Pritchard found 

 the hair cells, each one appeared to bear on its upper end four 

 or five stiff and relatively thick bristles — the auditory hairs. 



The hair cells are well-defined structures with flat, circular 

 tops, from which the stiff hairs rise, with large nuclei imbedded 

 in a granular protoplasm. 



The supporting cells are well developed, and extend from 

 the basement membrane quite to the surface of the sensory 

 ridge. 



After having left their channels in the lamina spiralis ossea 

 near its lower border, the nerves pierce the upper layers of the 

 lamina spiralis membranacea (basilar membrane), forming in so 

 doing the usual habenula perforata. The nerve fibrillae pass 

 upwards and outwards to the inner hair cells, and the latter or 

 outermost fibres, after crossing the canal of Corti, end in the 

 outer hair cells. The mammalian organ does not extend fur- 

 ther towards the apex of the cochlear tube than the lamina 

 spiralis ossea, and here the tube narrows, to suddenly enlarge 

 again into the lagenar pocket, which is curved around the open 

 space called the helicotrema, by means of which the two peri- 

 lymphatic spaces communicate. The lagenar sense organ in 

 Ornithorhynchus is found to be in structure much like the 

 maculae of the utriculus and sacculus of this animal's ear, but 

 more like the lagenar organ of the Sauropsida. The usual oto- 

 lith collection is present on the surface of the sensory hair field. 



