LECT. III.] EAR-DRUM OF MARSUPIALS. 71 



in the early state, and during growth become aggregated 

 laterally, so as to leave large fenestrae, or windows, 

 towards the mid-line. 



3. The temporal bone (squamosal) is hollow above 

 the hinge of the lower jaw, and this large air-cell 

 communicates with an extensive series of similar empty 

 spaces that arise primarily in the mastoid region, or 

 back part of the organ of hearing. 



4. The bony ring of the ear passage (external 

 auditory meatus and drum cavity), does not form all 

 the cavity, but a hollow shell of bone from the hinder 

 wing of the sphenoid (ala magna or alisphenoid) applies 

 itself in front and within, so as to form what is called 

 an " auditory bulk." Yet this bulla or bleb-like shell 

 of bone, a part that we do not possess, does not corre- 

 spond with that of the Cat, whose meatus-skeleton has a 

 large shell-like inner tympanic bone added to the usual 

 annulus, or ring-bone. 



5. The internal carotid artery does not enter the 

 skull, as in us, between the basal beam on one hand, 

 and the side-wings and petrous bones on the other, but 

 burrows through the basal beam, each branch appearing 

 in the seat of the turkish saddle, " sella turcica," 

 instead of burrowing the " petrosal," and then passing 

 through a "foramen lacerum," or ragged interspace 

 between that bone and the lateral parts of the skull. 



6. The malleus, or hammer-bone of the middle 

 ear, has a very large processus gracilis, which, be- 



