THE COCHLEA. 277 



them. The demonstration of these structures pre- 

 sents, however, the greatest possible difficulty. 

 Preparation 4. Sections of the cochlea. 



On account of the thinness of its osseous parietes, 

 the ease with which it may be obtained separate 

 from the surrounding bone, and its comparatively 

 large number of spiral turns, the cochlea of the 

 guinea-pig offers far greater facilities for study, and 

 especially for the preparation of sections, than that 

 of any other animal. The following is the mode of 

 finding and procuring it: In the recently-killed 

 animal the aperture of the mouth is prolonged back- 

 wards on either side, by cutting through the cheeks 

 and temporal muscles with strong scissors. The 

 lower jaw is then seized and forcibly torn away 

 from the rest of the head, so that the base of the 

 skull is exposed. Here will be seen on either side, 

 just behind the fossa for the articulation of the con- 

 dyle of the jaw, a large white bony projection the 

 tympanic bulla. This is not yet to be opened, but 

 the cartilaginous external auditory meatus is first cut 

 tli rough, and with the aid of bone-forceps or strong 

 scissors, the bulla in question, together with the 

 petrous bone to which it is attached, separated from 

 the rest of the skull. In a young animal this can 

 be readily effected, simply by inserting a strong 

 blunt instrument into the base of the skull just in 

 front of the bulla, and using it as a lever, raising 

 the bone and forcing it away from its attachments. 

 The bones of either side being thus removed, the ad- 

 hering soft parts are cleared away, and the bulla is 

 broken open at its most prominent part. On now 

 looking into the cavity there will be noticed, on one 

 side the delicate tympanic membrane stretching over 

 the end of the external meatus, with the handle of 

 the malleus attached to it, and on the opposite wall 

 a well-marked conical projection, indeed, its bony 

 wall is so thin that it is possible to count the number 

 of turns (four) which it presents. By cutting the 

 bulla round with strong scissors, the two parts one 

 24 



