202 COMPARATIVE ANATOMY. 



Reptilia and other Mammals ; and their auditory organ is similar 

 in many points to that of Crocodiles. At the same time nothing is 

 yet certainly known of the phylogeny of the Mammalian auditory 

 organ, concerning which further and more extended researches are 

 necessary. The cochlea here reaches its highest development, for 

 it grows into a long tube which becomes spirally coiled on itself : 

 in Man it forms nearly three coils, and in other Mammals from 

 one and a half (Cetacea) up to as many as four or more. 1 In this 

 spiral form of the cochlea, as well as in its more highly specialised 

 histological structure, lies the characteristic peculiarity of the 

 auditory organ of Mammals. The auditory nerve forms the axis 

 of the spiral (Fig. 166). 



In consequence of the large development of the cochlea, the 

 papilla acustica, or, as it is called in Mammals, the organ of Corti, 

 is drawn out to a considerable length, and the part of the wall of 

 the cochlea on which this lies is called the basilar membrane, 

 while the opposite wall is spoken of as the membrane of 

 Reissner (Figs. 169 and 170, B, E). These parts will be referred 

 to again later on. 



The aperture of communication between the pars superior and 

 pars inferior of the membranous labyrinth, that is, between the 

 sacculus and utriculus, is entirely obliterated in Mammals, the two 

 parts being only indirectly connected with one another by means of 

 the ductus endolymphaticus ; this bifurcates at its point 

 of insertion into the membranous labyrinth, one limb being con- 

 nected with the utriculus and the other with the sacculus (Fig. 

 167, at 2). 



The tympanic membrane is situated deep down in the external 

 auditory meatus, and separates the latter from the tympanic cavity. 

 In place of the single bony columella of the Sauropsida there is in 

 Mammals a chain of three auditory ossicles, articulated with 

 one another, and extending between the tympanic membrane and 

 the fenestra ovalis. These are, the malleus, the incus, with its 

 orbicular apophysis, and the stapes, besides which there is often 

 a bony (interhyal) rudiment in the tendon of the stapedius muscle. 

 The malleus corresponds to the articular element of the mandible 

 of lower Vertebrates, 2 and the incus to the quadrate, the former 

 arising by a constriction of the proximal end of Meckel's cartilage, 

 which extends through the so-called Glaserian fissure into the 

 tympanic cavity. As in the Sauropsida, the stapes corresponds to 

 the upper element of the hyoid arch (pharyngohyal or hyo- 

 mandibular of Fishes). The fact that in some cases the stapes 



1 In the Rabbit there are two and a half, in the Ox three and a half, in the Pig 

 almost four, and in the Cat three coils in the cochlea. In other types the cochlea, as 

 well as the sacculus and all parts of the pars superior of the membranous labyrinth, 

 vary considerably both in form and arrangement. 



2 Cp. the chapter on the skull, and Fig. 67, in which the mode of development of 

 these parts is shown. 



