AUDITORY ORGAN 303 



The original aperture of communication between the pars 

 superior and pars inferior of the membranous labyrinth that is, 

 between the sacculus and utriculus, is 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. 

 215) ; while its upper end perforates the inner wall of the carti- 

 laginous or bony auditory capsule, passes into the cranial cavity, 

 and terminates by an expanded extremity (saccus endolymphaticus) 

 in the dura mater. Osmosis can thus occur between the lymph 

 contained in the endolymphatic and epicerebral lymph -spaces 

 respectively. 



The tympanic membrane becomes secondarily situated deep 

 down in the external auditory meatus, and an important difference 

 is thus seen as compared with the Amphibia and most Sauropsida. 

 The tympanic cavity and Eustachian tube are well developed, and 

 in place of a single bony columella there is a chain of three 

 auditory ossicles, articulating with one another and extending 

 between the tympanic membrane and the fenestra vestibuli. These 

 are the malleus, the incus with its orbicular apophysis, and the 

 stapes. 1 



Two striated muscles are present in connection with the middle 

 ear. The phyletically older stapedius arises from the wall of the 

 tympanic cavity and is inserted into the stapes, serving to keep the 

 membrane of the fenestra vestibuli stretched. It is supplied by 

 the facial nerve and corresponds to the dorsal portion of the deep 

 constrictor inserted on the hyoid in Fishes, from which the 

 hinder belly of the biventer of Mammals also arises. A tensor 

 tympani, supplied by the mandibular division of the trigeminal and 

 derived from the system of the adductor mandibulae (pars 

 pterygoidea), 2 also arises from the wall of the tympanic cavity, and 

 is inserted on to the manubrium of the malleus, serving to stretch 

 the tympanic membrane. 



As already mentioned, the form of the membranous labyrinth 

 is repeated by its enclosing cartilaginous or bony capsule, which 

 forms, so to speak, a sort of cast around its individual parts. In 

 Mammals the skeletal labyrinth becomes ossified before any other 

 part of the skull, and is incompletely divided into two parts 

 enclosing the utriculus and sacculus respectively. With the latter 



1 Cf. Fig. 95, in which the mode of development of these parts is shown. 

 There is often also a bony (interhyal) rudiment in the tendon of the stapedius 

 muscle. 



2 In Man the tensor tj'mpani is from the first connected with the tensor veli 

 palatini muscle. In Ornithorhynchus it has a double origin, one part being 

 continuous with the pharyngeal muscles and the other arising independently. A 

 stapedius muscle is wanting in Ornithorhynchus and Echidna, and a tensor 

 tympani in Mauis ; and in all these three animals the tympanic cavity is sub- 

 divided into an upper and a lower portion by a horizontal septum of connective 

 tissue. 



