42 Praetium VI. The Tympanum. 



29. The Tympanodisk (membrana tympani). Trim the remnant of 

 the external auditory meatus (PI. II) close to the bone. Hold the head 

 so that the light enters it and note that it is closed by a membrane, 

 obliquely placed, and crossed by a light bar. The membranous septum 

 is the TYMPANODISK, or MEMBRANA TYMPANI, and the bar is the handle 

 of the MALLEUS, one of the three ossicles^of the ear. 



30. The Tympanum (middle ear or drum of the ear). Ventrad of 

 the meatus is a rounded elevation, the tympanic (or auditory) bulla (PI. 

 XVI). The ventro-caudal part is a thin shell of bone which may be opened 

 with the nippers or a stout pocket-knife. The considerable cavity is lined 

 by a delicate mucosa. 



a. Remove the caudal wall as completely as possible ; look in at the 

 caudal end and note a semilunar orifice leading cephalad. Just mesad of 

 this orifice is a circular depression, the FENESTRA ROTUNDA. 



b. Pass a probe very carefully into the semilunar orifice, looking at 

 the same time into the meatus ; the probe will be seen through the trans- 

 parent tympanodisk. 



31. Opening the Tympamim Proper. With nipper and coarse scis- 

 sors cut away the thin septum between the two cavities ; avoid injuring 

 the tympanodisk as long as possible, at any rate until there is recognized 

 the attachment of the handle of the malleus to its ental surface. Then 

 the margins of the cavity may be nipped away so as to expose it com- 

 pletely. 



32. The Auditory Ossicles. The long handle of the malleus forms 

 an angle with its head. Attached to an intervening neck is the short 

 tendon of an almost spherical muscle, the TENSOR TYMPANI. 



a. Move the handle of the malleus and note the communication of 

 the movement to two other bones, the INCUS and STAPES and the attach- 

 ment of the "foot" of the latter at the FENESTRA OVALIS. 



b. Extract the ossicles and examine with the magnifier, if possible 

 while resting on a dark surface. 



33. The Eustachian Tube. From the pharyngeal orifice ( 28) pass 

 the tracer cautiously dorso-caudad toward the tympanum, keeping the 

 concavity of the tracer ventrad ; the point will presently enter the tym- 

 panum at the side of a projecting shelf of bone. 



2 34. The Semicircular Canals. These and the other parts of the labyrinth (ental or 

 internal ear) are inclosed in dense bone. With fine nippers the bony tube containing one 

 of the canals may be opened, but the parts are too small in the cat for examination in this 

 connection ; see Anatomical Technology, 529-533, Fig. 127. 



35. The Nasal Cavity. With the scalpel cut off the end of the 

 nose close to the bone ; note the mesal cartilaginous NASAL SEPTUM ; in 

 PI. XVII the septum has been removed. At the sides of the septum are the 

 convoluted cartilaginous continuations of the TURKINALS or turbinated 

 bones. 



a. Upon some of these are distributed the olfactory nerves ; they are more complex 

 and abundant in the cat and especially in the dog than in man. 



b. At either side of the septum introduce the probe and push it 

 caudad, keeping close to the floor of the nasal cavity, and it will emerge 

 in the pharynx on the same side. 



36. The Lachrymal Duct. This has been mentioned in 5 ; its 

 nasal orifice is at a point ventrad of the M of the abbreviation O. M.vlrb. 

 in PI. XVII, but time and skill are required for tracing it. 



