PLATANISTA. 517 



to the fenestra rotunda. Tracing the extent of the cochlea from the somewhat 

 oval apical chamber, it would appear to consist of a little more than one and a 

 half turns. The canal of the cochlea, in transverse section, is seen in its first coil 

 from the vestibule to be divided into the two distinct portions, the scala vestibuli 

 and scala tympani, in a more marked manner than in other mammals, by the 

 existence of a prominent osseous ridge, or almost lamina, along the inner wall of the 

 periphery of the cochlea opposite to the lamina spiralis, the two being separated 

 from each other by only a narrow space. This ridge begins at i\\e fenestra rotunda, 

 and is almost lost beyond the first tm^n of the canal. The lamina spiralis does not 

 reach the apex of the cochlea, where there is a considerable chamber with perfectly 

 smooth walls, and instead of terminating in a hamular process, it ends in a rounded 

 extremity. It is much striated, indicating the tracts of the nerves ; and alono- its 

 under-surface, in close relation to the modiolus, runs a wide spiral cylindrical canal 

 with very delicate walls, its outer and inner walls being pierced by many foramina, 

 the remainder being smooth. Its upper wall is formed by the lamina spiralis 

 itself, and its inner wall by the modiolus. It commences at the foramen rotundum, 

 and the other canal, beginning at the fenestra rotunda, runs below the osseous 

 ridge of the periphery of the cochlea, and is partly membranous, and it is lost 

 about the end of the first tm-n. It will be observed (PL XLI, figs. 1 and 2) that 

 the coils of the cochlea are separated from each other by a considerable osseous 

 interspace. 



Semicircnlar canals (Plate XLI, figs. 1 to 12). — The characters and position 

 of the semicircular canals and their small size relatively to the cochlea, in the Ceta- 

 cean genera Platanista, Orcella, Glohiceplialus and Monodon, are well brought out 

 in the comparative series of ear-labyrinths figured, natural size, on Plate XLI, with 

 the semicircular canals themselves magnified twice their natural size. 



In connection with these figures, I have to mention that they are exact 

 reproductions of a series of metal moulds made from the ear-labyrinths of these 

 genera by my accomplished fellow- student and friend Professor Crum Brown, whose 

 ingenious researches' into the relations and functions of the semicircular canals are 

 now well known to every physiologist. 



In the plate in question, it will be observed that this blind dolphin has semi- 

 circular canals greatly exceeding in size Glohiceplialus, an animal more than double 

 its dimensions, and that the same holds good of these structures and the semicii'- 

 cular canals of Monodon, a whale which attains to 16 feet in length. These casts also 

 bring out this ckcumstance that the semicircular canals of Orcella are very much 

 smaller than those of this dolphin, although the animals are of nearly equal size ; 

 but whether other Cetacea of the same dimensions as Orcella have similarly- sized 

 canals I am not in a position to say. 



Although Orcella and Glohiceplialus are two forms allied to each other, it is 

 important to note that the semicircular canals of the latter whale, an animal twice 



• Proc. Roy. Soc. Edin., 1874, p. 255; Jour. Anat. Pliys., 1874, p. 327; see also MaoL Lehre v. d. Beweg. 

 Empfind., 1875 ; Breuer, Wien. Med. Jahrb., 1874, p. 72 ; 1875, p. 87. 



