150 



Dr. U. Pritchard. 



[Dec. 9, 



Comparison with Typical Mammals and Birds. 



In general form the duckbill's cochlea closely resembles that of the 

 bird, and is very different to the spiral cochlea of the ordinary 

 mammal. The two first differ, however, in that the duckbill's is 

 more curved, and curved outwards instead of inwards, as in the bird. 

 The enlarged apex of the former is rounded, that of the bird oval. 

 The typical mammalian cochlea tube differs from that of the duckbill, 

 in being spiral instead of merely curved, in tapering from commence- 

 ment to apex, and in being much longer. Lastly, the axis of the 

 spiral cochlea is horizontal, whereas that of the curved one is vertical. 



The Internal Arrangement and Minute Structure of the Duckbill's Cochlea. 



The interior of the tube is divided horizontally into two scalar by a 

 partition, the inner portion of which is thick and bony (lamina 

 ossea) ; the outer, thin and membranous (lamina membranacea) ; a 

 third scala is formed by a delicate membrane (membrane of Reissner) 

 proceeding from the upper surface of the lamina ossea to the inner* 

 wall of the tube. 



The upper and larger division is the scala vestibuli, and this com- 

 municates posteriorly with the vestibule ; the lower is the scala tympani. 

 These two are united at the apex of the cochlea by means of an oval 

 opening, helicotrema. The third, a small triangular tube, is the 

 ductus cochlea?, or scala media, and this constitutes the membranous 

 labyrinth ; it contains the endolymph, and is entirely separate from 

 the other two scala?, which contain the epilymph. The ductus cochlea? 

 is lined with epithelium ; the scala? vestibuli and tympani with 

 endothelium. 



The lamina ossea is a wedge-shaped mass of modified bone attached 

 to the lower part of the outer wall and the outer part of the floor of 

 the tube. Its inner free margin presents a deep groove (marginal 

 sulcus), the lower lip of which projects further inwards than the 

 upper. The lamina ossea does not extend to the apex of the cochlea, 

 and thus allows of the communication between the scala? vestibuli and 

 tympani. 



The ductus cochlea? is triangular in section, the floor is formed by 

 the inner portion (limbus) of the lamina ossea and a strong membrane 

 (membrana basilaris), which stretches from the lower lip of the sulcus 

 to a mass of connective tissue (ligamentum cochlea?) adherent to the 

 inner wall of the cochlea. The inner wall of the ductus is formed of 

 this ligamentum cochlea? ; and the outer wall, or sloping roof, by the 

 delicate membrane of Reissner, which springs from the upper surface 



* In describing the position of the parts in the duckbill's cochlea, the median 

 line of the body is taken as the centre ; in the spiral cochlea, the modiolus or axis. 



