146 ANIMAL BIOLOGY. [Part I. 



inner limb of the V- sna pecl pterygoid processes of the ali- 

 sphenoids, form the side-walls and lateral portions of the 

 roof. A horizontal shelf formed by palatine processes of 

 maxillae (pa. p. mx.) and palatines (pa. p. pa.) compose the very 

 incomplete bony floor. [This bony palate is much more 

 complete in the cat or dog.] In it are the very large anterior 

 palatine foramina (a. p. /.), and the small posterior palatine 

 foramina (p. p. /.). 



(3.) The Auditory Chamber. The periotic fits into the side-walls 

 of the brain-case just in front of the ex-occipital. It is very 

 irregular in shape, and differs in bone-density in its anterior 

 (petrous) and posterior (mastoid) portions. The latter is seen on 

 the exterior (45, per.) behind the tympanic bulla. On its inner 

 side are two depressions, one larger and blind, the floccular fossa 

 (44, A., /.), for the lodgment of the flocculus of the cerebellum 

 (see p. 217); the other smaller, the internal auditory meatus, 

 with nerve-foramina for the seventh and eighth nerves. Exter- 

 nally there are two openings leading into the cavity of the 

 bone, the fenestra ovalis before and the fenestra rotunda behind. 

 They are best seen on removing the bulla. Within the bone 

 are lodged, in the living rabbit, the essential parts of the organ 

 of hearing. In the embryo of mammals the periotic ossifies 

 from three centres (Fig. 44, C.), and may thus be regarded as 

 due to the coalescence of three bones, the prootic (pr. o.), epiotic 

 (ep. o.), and opisthotic (op. o.), which have the relative positions 

 and the relations to the fenestra ovalis (/. o.) and fenestra 

 rotunda (/. r.) diagrammatically shown in Fig. 44, C. (p. 140), 

 the prootic being anterior. 



External to the periotic, covering all its petrous portion, is 

 the tympanic, of which there are two well-marked divisions, a 

 swollen ventral portion, the bulla, and a spout-like dorsal pro- 

 longation, the neck (Fig. 45, ne.). 



Within the cavity of the bulla are several small bones, the 

 auditory ossicles. In the fresh state membranes are stretched 

 across the fenestra ovalis and rotunda, and a large tympanic 

 membrane is attached to a raised ridge which marks the junction 

 of the swollen and tubular portions of the tympanic. Attached 



