II 



SENSIBILITY OF THE INTERNAL ORGANS 113 



cochlea gradually acquires a spiral arrangement. Finally in 

 mammals it reaches its greatest, development in the form of a 

 long twisted tube, with one and a half to four or more spiral 

 turns. The cochlea, with the nervus cochlearis which forms ;i 

 delicate end-organ within it, undoubtedly represents the <rg;m 

 of hearing, as will be fully discussed below. 



The membranous labyrinth with the terminations of the 

 vesiihular branch of the 

 eighth nerve forms an- 

 other delicate sense- 

 organ which phylogeu- 

 etically represents the 

 tirst stage in the differ- 

 entiation and perfecting 

 of primitive cutaneous 

 sensibility. It is con- 

 tained within the bony 

 labyrinth hollowed out 



v 



of the petrous bone, 

 the form of which is 

 clearly seen from the 

 models obtained on 

 pouring molten metal 

 into the cavity of the 

 labyrinth (Fig. 44). The 

 formation of the mem- 

 branous labyrinth and 

 its different parts is 

 shown in Fig. 45. The 

 membranous labyrinth 

 filled with endolyuiph is 

 contained in the cavity 

 of the vestibule, which 

 in its turn is filled with 

 perilymph. Fig. 46 

 gives some idea of the topographical relations of the labyrinth 

 with the tympanic cavity and the external ear. 



The semicircular canals are orientated according to the three 

 planes of spatial dimension. On each side there is an external 

 canal, an anterior canal, and a posterior canal. As appears 

 plainly in Fig. 47, the tw T o outer canals lie almost exactly in the 

 same horizontal plane; the planes of one posterior canal and the 

 anterior canal of the opposite side are almost, exactly parallel, 

 and form with the median plane an angle of about 45. The 

 six semicircular canals thus form together three planes, one 

 horizontal and two vertical, which are perpendicular to each 

 other and to the horizontal plane, so that they are orientated 



VOL. iv i 



Fio. 47. Diagram of orientation of .semicircular canals in 

 pigeon seen from liehind ami within after opening tin- 

 skull. (Ewald.) The anterior canal lies in plane A ; the 

 external canal in plane !'. ; the posterior canal in plane /'. 



