372 ELEMENTARY PHYSIOLOGY less. 



the front wall of the drum to the malleus. Each of the 

 muscles when it contracts tightens the membrane to which 

 it is thus indirectly attached, the tensor tympani, the 

 membrane of the drum, and the stapedius, the membrane 

 of the fenestra ovalis. The effect of thus tightening the 

 membrane is probably to restrict the vibrations of the 

 membrane, at least as far as concerns grave, or low-pitched 

 sounds, but the complete action of these muscles is too 

 intricate to be dwelt on here. 



(v) The Inner Ear consists, substantially, of a very 

 peculiarly-formed membranous bag. This bag, when the 

 ear first begins to be formed, is a simple round sac, but 

 it subsequently takes on a very complicated form, and 

 becomes divided into several parts, which receive special 

 names. It is lodged in a cavity of correspondingly 

 intricate shape, hollowed out of a solid mass of bone 

 (called from its hardness petrosal), which forms part of the 

 temporal bone, and lies at the base of the skull. The sac, 

 however, does not completely fill the cavitj', so that a 

 Rpace is left between the bony walls and the contained 

 sac. This space, otherwise continuous all round tlie sac, 

 is interrupted at certain places only where the mem- 

 branous sac is attached in the bony walls, and contains a 

 fluid provided by the lymphatics of the neighbourhood, 

 and called perilymph. 



The membranous sac, the walls of which consist chiefly 

 of connective tissue, is lined by an epithelium, and 

 contains a fluid of its own called endolymph. The 

 perilymph, it will be understood, is quite distinct fi-om 

 the endolymph, the two fluids being separated by the 

 walls of the membranous sac. 



Over a great part of the interior of the membranous sac 

 the epithelium is simple in character, but at certain places 

 to be presently described it assumes special features. 



So mi^ch is true of the sac as a whole, l)ut the different 

 portions of it have very different functions. The general 

 shape of the sac is shown diagrammatically in Fig. 124. 



