44 PHYSIOLOGICAL SERIES. 



tilled with air) is connected with the tympanum ; that of 

 the right side has been opened. Red bristles show the 

 course of the Eustachian tubes and their openings into the 

 posterior ends of the bulUe. A cavity in the base of the 

 zygoma and mastoid process of the squamosal opens into 

 the tympanum ; it is partly closed by the skin on the dorsal 

 wall of the meatus ; a black bristle has been passed into it 

 from the bulla. These air-chambers render the bone light. 

 and may act as resonating chambers associated with 

 hearing. 



Paulli, Morph. Jahrb., Bd. xxviii. 1899, p. 147. 



A. 198. Cranium of a Chinchilla (Chinchilla lanlyera}. Each 

 tympanum has two bullpe one ventral and one dorsal. 

 The ventral is of large size ; it extends backwards and 

 upwards to half the height of the skull : red bristles are 

 inserted into the Eustachian openings. The dorsal bulla. 

 indicated by a black bristle, commences as a diverticulum 

 of the tympanic roof close to the membrana tympani : it 

 passes upwards on the inner side of the meatus, forms a 

 flattened projection on the dorsal surface of the skull, and 

 extends backwards and downwards to meet the ventral 

 bulla, from which it is separated by a bony septum. The 

 dorsal bulla is said to be formed in the periotic bone ; the 

 ventral either in the tympanic or in a special cartilage. 



Presented Inj F, G. Parsons, Es<j. 



A. 199. Left half of the cranium of a Horned 8heep (Ons 



showing the air-filled cavities in the bones that communicate 



O 



either with the nasal chamber or tympanum. The frontal 

 sinus and its communication with the nasal cavity is indi- 

 cated by a black rod ; this sinus extends into the horn-core. 

 A blue rod has been passed into the middle meatus of the 

 nose from the large cavity in the superior maxilla (antrum 

 of Highmore). A red rod indicates the Eustachian opening 

 into the tympanum. The floor of the tympanum is ex- 

 panded into a bulla of less size than in the Carnivora, and 

 possesses no septum. 



The bullse may serve as resonating chambers connected 



