128 VERTEBRATE ZOOLOGY 



The latter is a large T-shaped membrane bone which covers the 

 entire ventral portion of the primitive cartilaginous cranium and 

 overlaps the ethmoid ; its lateral portions extend to the auditory 

 capsules. At the sides of it, back of the ethmoid, will be seen the 

 primitive cartilage. The large foramen of the optic nerve will be 

 seen in the side of the cranium between the frontoparietal and 

 the parasphenoid. 



The Special-Sense Capsules. These are the auditory capsules 

 at the hinder end of the skull, and the nasal capsules at its forward 

 end. The optic capsules do not ossify; they are largely mem- 

 branous, with delicate cartilaginous plates in the sclera. 



The auditory capsules are fused with the sides of the cranium 

 proper and consist largely of cartilage. On the ventral side of the 

 skull the lateral projections of the parasphenoid bone cover them. 

 On the dorsal and anterior sides a cartilage bone called the prootic 

 is present ; it will be seen abutting the hinder part of the fronto- 

 parietal bone. Between the prootic and parasphenoid bones is 

 the large foramen of the trigeminal nerve. 



Ventral to the prootic on the side of the skull is a depression ; 

 at the bottom of this is a large hole called the fenestra ovalis, which 

 looks inside the auditory capsule. The depression is the tympanic 

 cavity, the middle ear of higher vertebrates, which in the fresh 

 skull is covered laterally by the tympanic membrane, or ear drum. 

 The fenestra ovalis is closed by a minute cartilage called the 

 operculum, which, in turn, is joined with the tympanic membrane 

 by a small bony and cartilaginous rod called the columella. These 

 two small structures, like the ossicles of the mammalian ear, con- 

 vey the sound waves from the tympanic membrane to the inner 

 ear. Skulls from which the tympanic membrane has been removed 

 have often lost them. 



The two nasal capsules lie side by side, fused with the anterior 

 end of the cranium proper, and are also composed largely of carti- 

 lage. The ringlike ethmoid bone, which, as we have seen, forms 

 the anterior end of the brain case, also forms the posterior end of 

 the nasal capsules. Two pairs of membrane bones are present in 

 these capsules, the dorsal nasals and the ventral vomers. The 

 former are a pair of large bones which lie in a transverse position 



