THE BULLA TYMFAISICUM. 185 



O. pmx., Os praemaxillare.— Prsemaxillary or intermaxillary bone. 



§ 543. Pre. par., Processus paroccipitalis — s. paramastoideus — Paramastoid or 

 paroccipital process. — This is a curved shelf-like projection of the exoccipital which abuts 

 against the caudal end of the bulla. 



Pre. pro., Processus mastoideus — s. pars mastoidea. — Mastoid portion of the p&ri- 

 oticurii. 



Pre. z., Processus zygomaticus. — Zygomatic process of the temporal bone. 



Pre. po.. Processus postorbitale. — Postorbital process of the frontal bone. 



§ 543. Fig. 68— Preparation.— The soft 

 parts were removed ; then, while the skiill 

 was yet moist, the ventral face of the buUa 

 was ground off .on- a fine emery-stone. 

 It might be removed with a watch-spring 

 saw or on an ordinary grindstone. In 

 grinding, the pressure should be only 

 moderate to avoid breaking the delicate 

 septum. 



§ 544. Bulla tympanica — The auditory or tympanic 

 bulla. — The bulla is a hollow subspherical part of the 

 0. tympanicum enclosing the tympanum or middle ear. 

 A delicate bony septum divides the cavity into two un- FiG. 58. — Ventbo - lateral 

 equal parts. This septum arises from the floor, and View of the Left BuLLji. 

 extends dorsad, but leaves a space over the fenestra ro- and Adjacent Pakts ; x 1.5. 

 tunda which puts the two cliambers in communication. 



In the lateral aspect of the bulla is the external auditory meatus (M. a. e.) ; attached to 

 the ectal part of this is the external ear, and to the ental part the Menibrana iympani or 

 ear drum, which completely separates the tympanum or middle ear from the exterior of 

 the body. Flower, A, 110, and 26, 4 ; Huxley, B, 249 ; Straus-Durckheim, A, I, 409. 



Cd. oe., condylus occipitalis. — Occipital condyle. 



§ 545. Cn. eu., Canalis Eustachiana — Eustachian canal. — This is a short bony tube 

 leading from the tympanum to the ventral surface of the skull. Flower, A, 111. 



Fm. m., Foramen magnum, az. 



Fm. cd., Foramen condylare. — Condylar foramen. 



Fm. ov., Foramen ovale. 



Fs. mnd., Fossa mandibularis. — Mandibular or glenoid fossa (Fig. 57), 



Ft. rt., Fenestra rotunda (Fig. 57). 



Ft. ov., Fenestra ovalis. 



M. a. e., Meatus auditorius externus.— External auditory meatus. 



O. boc, Os basioccipitale, az. — Basioccipital bone. 



Pre. z., Processus zygomaticus. — Zygomatic process of the temporal bone. 



Spt. tym.. Septum tympanicum.— The bony partition dividing the interior of the 

 bulla into two unequal chambers. 



§ 546. Fig. 59 — ^Preparation. — The soft parts were removed 

 (§ 250, B), and then the section was made with a watch-spring saw 

 while the skuU was 'still moist. The section was made about 2 mm. 



