430 MANUAL OF ELEMENTARY ZOOLOGY 



is the mastoid portion. Against the lower part of the periotic is placed 

 a thin membrane bone shaped like a flask with a gap on one side, the 

 gap being turned towards the periotic. This is the tympanic bone. 

 The body of the flask, or bulla, encloses the tympanic cavity, and the 

 neck leads upwards and outwards from the drum to the ear opening, 

 the passage it encloses being known as the meatus auditorius extemus. 

 At its inner end is a ring which marks the position of the drum in life. 

 The inner wall of the tympanic cavity is formed by the periotic bone, 

 and on it may be seen two gaps, ike fenestra ovalis and behind this the 

 fenestra rotunda. In life a chain of three little cartilage bones, the 

 malleus, incus, and stapes, connect the fenestra ovalis with the drum 



i/mx. 



Fig. 314. — A side view 01 a rabbit's skull. — From Thomson. 



Pmx., Premaxilla ; Na., nasal ; Fr., frontal ; Pa., parietal; Sq?, squamosal; S.O., 

 supraoccipital ; Per., periotic ; 71, tympanic (the reference line points to the 

 bony external auditory meatus, beneath it lies the inflated bulla) ; P.O., par- 

 occipital process. 



in the same way as the columella auris of the frog. These bones 

 belong in reality to the visceral arches. 



The part of the skull in front of the cranium is known as the facial 

 region. It consists of the nasal capsules and certain of the bones of 

 the upper jaw, and we have seen that it is bent downwards at an angle 

 of 60 with the cranium. The nasals are elongated membrane bones 

 which form the roof of the nasal cavities, uniting by a suture with the 

 frontals behind. The mesethmoid is a median, vertical plate of 

 cartilage extending forward from the cribriform plate and separating 

 the nasal cavities. The vomers are a pair of slender, blade-like 

 bones, fused with one another along their lower edge, which enclose 

 between them the lower edge of the mesethmoid cartilage and 

 thus with it support the nasal septum (p. 437). Behind they send out 

 flanges or "wings " towards the sides of the nasal cavity, so as to form 



