428 THJ-. DOC. I'Jivi. i;m chokjjata 



parietals ; and the frontal ring of pre- and orbitosphenoids 

 and frontals. The interparietal and parietals are raised into 

 a median sagittal crest, to which muscles are attached. The 

 frontals are pressed inwards below to make two hollows, the orbits, 

 into which fit the eyes, and on its upper edge each frontal has 

 a conspicuous postorbital process which juts out over and behind 

 the eye. The orbitosphenoid may always be recognised by the 

 fact that it entirely surrounds the optic foramen, which is the 

 most anterior large hole in the orbit. The alisphenoid, which is 

 behind the orbitosphenoid, may be recognised because it bears 

 near its surface a conspicuous tunnel, the alisphenoid canal, 

 through which a seeker may be passed from one end to the 

 other without going into the cranial cavity. 



As we have seen, the auditory capsule makes part of the side 

 wall of the brain-box. It ossifies from three centres, but in the 

 adult only a single cartilage bone, the periotic or petrosal, can 

 be recognised. Most of it, which encloses the inner ear, can only 

 be seen in a sectioned skull, but a small part, which bears a 

 projection, the mastoid process, is visible externally. The mem- 

 brane bone of the auditory capsule is the squamosal, one of the 

 largest bones in the skull, which completes the side wall between 

 alisphenoid and the exoccipital. It has a large outwardly and for- 

 wardly projecting zygomatic process ; the upper part of this 

 forms part of the zygomatic arch, a half-hoop of bone which runs 

 outside the jaw muscles below the eye, and the lower forms a 

 cylindrical hollow for articulating with the lower jaw. Associated 

 with the auditory capsule of mammals are some other bones, 

 which, although they originally belonged to the ventral part 

 of the skull, have become intimately associated with the ear. 

 The tympanic is visible on the outside, between the mastoid 

 process and the squamosal, as a flask-shaped bulla. Inside the 

 lower end of the neck of this is a ring, where in life the ear drum 

 is fixed ; the neck leads up from this to the surface of the head, 

 forming a passage, the external auditory meatus. The body of the 

 bulla encloses the tympanic cavity, the posterior wall of this being 

 made by the petrosal. In this are two gaps, the fenestra ovalis 

 and fenestra rotunda (p. 469) and from the former of these a 

 chain of three ear ossicles runs to the ear drum. These are, from 

 the drum inwards, the malleus, incus, and stapes. 



The nasal capsule is partially ossified to form the front part 

 of the mesethmoid, which we have already mentioned, but much 



