AUDITORY ORGAN 231 



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 in- 

 sertion into the membranous labyrinth, one limb being connected 

 with the utriculus and the other with the sacculus (Fig. 179) ; while 

 its upper end perforates the inner wall of the cartilaginous 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 en- 

 dolymphatic and epicerebral lymph-spaces respectively. 



The tympanic membrane is secondarily situated deep down in the 

 external auditory meatus, and thus an important difference is seen 

 between the Amphibia and Sauropsida on the one hand, and the 

 Mammalia on the other. The tympanic cavity and Eustachian tube 

 are well developed, and in place of the single bony columella of the 

 Sauropsida there is a chain of three auditory ossicles, articulating 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}- 



The stapedius muscle arises from the wall of the tympanic cavity, and i.s 

 inserted into the stapes, serving to keep the membrane of the fenestra 

 ovalis stretched. It is supplied by the facial nerve and corresponds to a 

 specialised portion of the hinder belly of the biventer, and can be traced back 

 as far as Fishes. A tensor tympani supplied by the mandibular division of the 

 trigeminal and derived from the internal pterygoid muscle (primarily from 

 the masticatory muscles of Fishes) also arises from the wall of the tympanic 

 cavity, and is inserted into the manubrium of the malleus, serving to keep 

 the tympanic membrane stretched. Both these muscles are composed of 

 striated fibres. 



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. Thus 

 it is usual to speak of a cartilaginous or bony labyrinth as distin- 

 guished from the membranous labyrinth enclosed within it, the 

 two being separated by the perilymphatic cavity. 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 part is 

 connected the bony cochlea, the axis of which lessens in size from 

 base to apex (Fig. 185), and round it a bony lamella (lamina 

 spiralis ossea) winds in a spiral manner ; this extends into the 

 cavity of the coils of the cochlea without quite reaching the 

 opposite wall (Figs. 185 and 186), being continued outwards by 

 two laterally-diverging lamellae, mentioned above as the basilar 



1 Cp. p. 100 and Figs. 80 and 233, 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. 



