396 SPECIAL PHYSIOLOGY. 



pani muscle, according to Toynbee, is inclosed in a tubular ligament, 

 which, he supposed, keeps the tympanum in a state of medium tension, 

 the tensor tympani only being called into play when the tension of the 

 membrane is increased. When the tensor tympani acts, the head of 

 the long leg of the incus is drawn inwards, so that the base of the 

 stapes, which is articulated with it, must also advance towards the 

 inner wall of the tympanum, and so press in the membrane of the fen- 

 estra ovalis. Fick has proved this, by direct observation. The third 

 muscle, called the stapedius, is inserted into the stapes; it is generally 

 regarded as a tensor of the membrane of the fenestra ovalis, but, by 

 some, it has been described as relaxing that membrane. It is the 

 smallest muscle in the body. The tensor tympani is supplied by a 

 nerve from the otic ganglion, the laxator, it is said, by the chorda 

 tympani nerve, and the stapedius by a branch of the facial nerve. 



Below and rather behind the fenestra ovalis, on the inner wall of the 

 tympanum, is another small rounded opening in the bone, called the 

 fenestra rotunda ; it is closed in the recent state, by a membrane. 



The mucous membrane lining the tympanum is thin, and, for the 

 most part, covered with ciliated epithelium ; it assists in closing the 

 two fenestrse, and serves to form the inner layer of the membrana 

 tympani. In the latter situation it is said to be destitute of cilia; 

 lastly, it is reflected over the little ossicles, and the tendons of their 

 muscles, and also over the chorda tympani nerve, which traverses the 

 tympanum. It contains no mucous glands, but is constantly moistened 

 with a yellowish fluid. In front, it is continuous with the ciliated mu- 

 cous membrane lining the Eustachian tube, and, through it, with that 

 of the upper part of the pharynx ; behind, it enters and lines the 

 mastoid cells. 



The internal ear, or labyrinth, Fig. 74, s, c, and Fig. 77, so called 

 from its complicated communications, contains the essential parts of 

 the organ of hearing, viz., the membranous labyrinth and the cochlea. 

 It consists of certain complex chambers and canals, each inclosing 

 membranous and fluid contents ; it is buried in the substance of the 

 petrous portion of the temporal bone, and communicates, but by closed 

 apertures, externally, with the middle ear, through the fenestra ovalis 

 and fenestra rotunda, and internally, with the internal auditory mea- 

 tus, which transmits the auditory nerve. The labyrinth consists of 

 three parts, named, respectively, the vestibule, the semicircular canals, 

 s, and the cochlea, c. 



The vestibule, see Fig. 77, the central chamber of the bony laby- 

 rinth, is of an oval shape, corresponds in position with the fenestra 

 ovalis and the base of the stapes, and communicates freely with the 

 semi-circular canals and the cochlea. It is the fundamental portion 

 of the labyrinth, and is the only part present in the lowest Vertebrata. 



The semicircular canals, s, are placed above and behind the vesti- 

 bule; they are three in number, and are designated, according to their 

 position, vertical, horizontal, and oblique. These canals are curved 

 bony tubes, about ^th of an inch in diameter, each having at one end 

 a dilated part, twice as wide, called the ampulla. As two of these 



