THE INTERNAL EAR OR LABYRINTH 771 



altogether. The stapedius muscle, therefore, serves the same purposes 

 as the tensor tympani, i.e., it accommodates the membrane of the fen- 

 estra ovalis to high sounds, and prevents those of unusual intensity from 

 reaching the internal ear. The motor fibers of this muscle are derived 

 from the facial nerve. 



Both muscles react in consequence of reflex stimuli which appear 

 to be derived from the auditory nerve, 1 whence they are transferred 

 in the medulla to the aforesaid motor paths. These stimuli arise at 

 the very beginning of the different sounds and subject these membranes 

 to constant changes. Some persons are capable of contracting the 

 tensor tympani voluntarily. 2 



CHAPTER LXIV 

 THE INTERNAL EAR OR LABYRINTH 



General Structure. The general cavity of the internal ear, or 

 osseous labyrinth is hollowed out of the petrous portion of the tem- 

 poral bone. It consists of three parts, namely, the vestibule, the 

 semicircular canals and the cochlea. It is lined throughout with thin 

 periosteum. This entire space is filled with a lymphatic fluid, called 

 the perilymph. Suspended in this fluid is a membranous reproduction 

 of the osseous labyrinth, which in turn is filled with "a lymphatic 

 fluid, called the endolymph. The outer surface of the latter keeps at 

 varying distances from the wall of the bony cavity. The space be- 

 tween them is occupied by perilymph and is transected by ligamentous 

 bands and fibers which hold the membranous labyrinth in place. In 

 the vestibular part of the osseous labyrinth, this membranous tube 

 shows two enlargements which are known respectively as the utricle 

 and saccule. The former is directly continuous with the membranous 

 tubes of the semicircular canals and the latter, with the membranous 

 canal of the cochlea. 



It will be brought out later on that the semicircular canals are con- 

 cerned solely with the sense of equilibrium, while the cochlea mediates 

 the sense of hearing. For the present, therefore, we must confine our- 

 selves to a study of the latter structure. The cavity of the internal 

 ear is separated from that of the tympanum by a bony wall, which is 

 perforated in two places to form the fenestra ovalis and the fenestra 

 rotunda. Both openings are closed by membranes, the outer surfaces 

 of which lie in contact with air, while their inner surfaces border upon 

 the perilymph of the labyrinth. It has also been pointed out that the 

 vibrations in air are eventually converted into vibrations of lymph at 



1 Henson, Pfliiger's Archiv, Ixxxvii, 1901, 355. 



2 Mangold, Pfliiger's Archiv, cxlix, 1913, 539. 



