LEPUS CUNICULUS 297 



well-marked parts, the petrous portion on the inside and the mastoid, 

 which is produced posteriorly into a mastoid process lying next to the 

 par-occipital process. An oval opening, the internal auditory meatus, 

 lies on the middle of the inner surface of the per-iotic bone ; into this 

 go the eighth nerve supplying the internal ear and the seventh nerve 

 which reaches the outside of the skull through the stylomastoid 

 foramen , as has been noted above . On the outer surface of the peri-otic 

 bone facing the tympanic cavity, and so not seen unless the tympanic 

 bulla is removed, are two small holes, the fenestra ovalis and fenestra 

 rotunda, which place the inner ear in communication with the 

 cavity. Within and stretching across the tympanic cavity itself 

 is a chain of small bones connected with the function of hearing. 

 The malleus is a small bone consisting of a blade-like portion, the 

 manubrium, which is attached to the tympanic membrane, and a 

 more solid body. The latter part articulates with the next bone in 

 the series, the incus, and this also has a tiny process, the end of 

 which fits on to a small bony disc, the OS orbiculare. The last 

 bone, the stapes, is shaped somewhat like a stirrup with a central 

 perforation. At one end it joins the os orbiculare and the other 

 is flattened to form a basal plate that closes the fenestra 

 ovalis. 



To turn now to the jaws, which although laid down in 

 cartilage in the embryo, are entirely replaced by membrane bones 

 in the adult. 



The pterygoids are thin lamellae of bone lying almost in the vertical 

 plane ; their bases lie upon the pre-sphenoid and basi-sphenoid, 

 laterally they pass dov/nwards on the inside of the alisphenoid to 

 which they are closely apposed and they project freely beyond this 

 for a short distance only. Thus in looking at the skull from the 

 side only the small triangular points of these bones are visible. 

 At their anterior end they are attached to the next bone in the 

 series, the palatine. This is an irregular and much larger bone. 

 Its lateral portion which abuts on to the pterygoid continues upwards 

 and outwards to form a considerable part of the antero-ventral 

 wall of the orbit, where it articulates with the alisphenoid behind, 

 and the orbito-sphenoid and frontal above. It stretches forward 

 to the lachrymal and contributes partly to the mesial margin of 

 the hinder opening of the infra-orbital canal. Later in its orbital 

 region it joins the maxilla. Near the line of junction with the 

 maxilla lie two small perforations in the palatine bone. The upper 

 of these leads into the narial passage and transmits the posterior 

 nasal branch of the trigeminal, while the lower is for the palatine 

 branches of the same nerve and the palatine artery which pass 

 through the posterior palatine foramen of the maxilla and several 



