ORGAN OF HEARING IN MAMMALIA. 241 



nearly a right angle, and is inserted into the handle of the malleus 

 below the long process. By the action of this muscle the handle 

 is drawn inward and forward, and the membrane attached to the 

 handle is also drawn inward and is stretched. Besides the tension 

 to which the membrana tympani is thus subjected, the base of the 

 stapes is forced against the vestibular fenestra in consequence of 

 the movement communicated by the head of the malleus to the 

 incus, which tends to press inward the long extremity of the 

 latter. The second muscle is the ' stapideus,' fig. 167, f: it arises 

 from a groove in the ' pyramid,' fig. 18-i, d: it is inserted into the 

 posterior and upper part of the head of the stapes by a slender 

 tendon, which issues by the aperture in the summit of the pyramid, 

 and proceeds downward and forward to its termination. 



The first effect of the action of this muscle will be to press the 

 posterior part of the base of the stapes against the vestibular 

 fenestra : at the same time the long branch of the incus will be 

 drawn backward and inward, and the head of the malleus being, 

 by this movement of the incus, pressed forward and outward, its 

 handle will be carried inward, and the membrana tympani thus 

 put on the stretch. On the other hand, the contraction of the 

 ' tensor tympani ' depresses the stapes and increases the tension 

 of the fenestral membrane. The cessation of muscular action 

 restores all the vibratile membranes to their state of indifference. 

 The incus, by its firm connection with the mastoid cells, its inter- 

 mediate position, and having no muscle inserted into it, must be 

 more limited in motion than the other two bones. 



The stapideus muscle receives a nervous filament from the facial 

 nerve. The tendinous insertion of the stapes is usually the seat of 

 ossification. These muscles have no homolo<mes in Vertebrates 



O 



devoid of tympanum and tympanic membrane : they are as purely 

 independent and superadded parts of the mechanism of that ad- 

 vance of the auditory organ, in Mammals, as are the ossicles they 

 move. 



The cartilage described by Meckel, and representing the man- 

 dibular haemal arch in the embrvo-skull, from the fibrous sheath 



/ 



of which are developed the ' tympanic ' at the upper and outer 

 part and the mandible at the lower and outer part, has no such 

 relation of a mould to the malleus. This ossicle, startino- as a 



o 



wart-like prominence from the wall of the tympanic cavity, is 

 precociously developed on the inner side of Meckel's cartilage, 

 early showing its long process above and quite distinct from that 

 cartilage or its capsule. The short crus of the ; incus ' has the 

 VOL. IT r. it 



