CHAP. XVIII.] 



THE INTERNAL EAR. 



71 



Fig. 133. 



short and a long cms : the former, sc, has a backward direction, and 



projects into the mastoid cells, the latter, Ic, descends to a level with 



the fenestra ovalis, bends inwards, and is tipped with a lenticular 



process, to which the head of the stapes 



is attached, a. The stapes, or stirrup 



bone, s, is almost sufficiently described 



by its name. Its construction is truly 



elegant. It has a head, neck, two 



branches, and a base. The last fits into 



the fenestra ovalis, to the margin of 



which it is attached, by membrane, so 



as to enjoy some freedom of motion. 



Its neck receives the insertion of the 



stapedius muscle. The chain of ossicles, 



nOW described. Stretches acrOSS the 



. , . 



tympanum by no means in a straight Head f the maiieus, beiow which is the 



. * -i constriction or neck. g. Processus gra- 



line, and. its parts are permitted to cms, or long process, at the root of which 



n . is the short process, h. Manubrium, or 



eniov some degree ot motion, not mere- handle, sc. short cms; and ic, long 



ill i 11 . . , crus of the incus. The body of this bone 



ly by the double lOint existing between is seen articulating with the malleus, and 



, , , , ' , . . . its long crus, through the medium of the 



them, but by the mode OI their attach- orbicular process, here partly concealed, 



. , _ a, with the stapes, s. Base of the stapes. 



at either end. Magnified three diameters. From Arnold. 



Ossicles of the left ear articulated, and 

 seen from the outside and below, m. 



These bones are moved by small muscles, two of which are not 

 disputed. These are the internal muscle of the malleus, and 

 the stapedius muscle. Each of these muscles consists of striped 

 fibres. 



The internal muscle of the malleus, or tensor tympani, occupies 

 the canal above the osseous portion of the Eustachian tube. It is 

 attached in front to the under surface of the petrous bone, and to 

 the cartilage of the Eustachian tube; it- proceeds backwards, and 

 ends in a tendon which turns abruptly outwards from the osseous 

 canal in which the muscle is lodged, and is inserted into the short 

 process of the malleus. It draws this part inwards, and thus 

 heightens the tension of the membrana tympani. An anterior mus- 

 cle of the malleus, or laxator tympani muscle, is described by many 

 anatomists as passing from the Glaserian fissure to the processus 

 gracilis. The stapedius muscle occupies the conical interior of the 

 pyramid ; its surface is aponeurotic, its interior fleshy, and it ter- 

 minates in a small tendon which emerges at the apex of the pyramid, 

 and then passes to be inserted into the neck of the stapes. In con- 

 traction it would fix the stapes by pulling its neck backwards. It 

 probably compresses the contents of the vestibule. 



Of the Internal Ear, or Labyrinth. This is the potential part 



