604 THE NERVOUS SYSTEM. 



to a stirrup. It consists of a head, which articulates with the 

 os orbiculare ; a neck, into which the stapedius muscle is inserted ; 

 and two crura, which are connected with the base, this being 

 attached by ligamentous tissue so as to close. the fenestra ovalis. 



Ligaments of the Ossicles. The ossicles are connected with one 

 another, and also with the walls of the tympanum, by ligaments 

 (Fig. 384). One of these, the anterior ligament of the malleus, 

 was at one time supposed to be a muscle and was described under 

 the name levator tympani. 



Muscles of the Ossicles. These are two in number tensor 

 tympani and stapedius. 



Tensor Tympani. This muscle lies in a bony canal which 

 is above the canal containing the Eustachian tube, and sepa- 

 rated from it by the processus cochleariformis, a thin plate of 

 bone. It has its origin from the petrous bone, the cartilaginous 

 portion of the Eustachian tube, and the bony canal in which it 

 lies. Its tendon enters the tympanum and bends at almost a 

 right angle around the end of the processus cochleariformis, and 

 is inserted into the mauubrium near the neck. Its nervous supply 

 is a branch from the otic ganglion. When this muscle contracts, 

 the membrana tympani is drawn inward and made more tense. 



Stapedius. This muscle arises from the interior of the pyramid 

 which is situated just behind the fenestra ovalis, and just below 

 the opening of the mastoid antrum, and its tendon passes out 

 at an opening in the apex ; it is inserted into the neck of the 

 stapes. Authorities do not agree as to the effect of the contrac- 

 tion of this muscle. Gray says that "it draws the head of 

 the stapes backward, and thus causes the base of the bone to 

 rotate on a vertical axis drawn through its own center ; in doing 

 this the back part of the base would be pressed inward toward 

 the vestibule, while the fore part would be drawn from it. It 

 probably compresses the contents of the vestibule." 



Sewall, in the American Text-Book of Physiology, says : " Con- 

 traction of the muscle would cause a slight rotation of the stapes 

 round a vertical axis, so that the hinder part of the foot of the 

 ossicle would be pressed more deeply into the fenestra, while the 

 remaining portion would be drawn out of it. Its action probably 

 reduces the pressure in the cavity of the perilymph, and thus is 

 antagonistic to that of the tensor tympani." The nervous supply 

 of this muscle is the tympanic branch of the facial, which reaches 

 the muscle through a canal in the pyramid which communicates 

 with the aquseductus Fallopii. 



Eustachian Tube (Fig. 385). Through this channel the tym- 

 panum is in communication with the pharynx. It is about 36 mm. 

 in length, and passes downward, forward, and inward. It begins 

 in the lower part of the anterior wall of the tympanum, and is 

 bony for about 12 mm. It then becomes cartilaginous, and 



