62 ANATOI^IY OE VERTEBRATES. 



tween the outer and inner tables.^ The owls have a similar de- 

 velopement of diploe : in most birds the free cranial wall is thin 

 and compact. The cavity is closely moulded to the brain, and 

 shows well-marked fossae for the cerebellum, medulla oblongata, 

 optic lobes, hypophysis, cerebral hemispheres, and, in Dinornis 

 and Apteryx, for the olfactory lobes. Some birds show also a 

 depression upon the petrosal, which is deep in the Heron. In 

 Dinornis an upper transverse ridge divides the pros- from the 

 ep-encephalic compartment, and a lower one divides the pros- 

 from the mes-encephalic compartment, which ' tentorial ' ridges, 

 being on nearly the same vertical parallel, almost equally bisect 

 the cranial cavity into a wider front and narrower hind division. 

 The roof of the prosencephalic compartment sinks a little into the 

 interspace of the hemispheres, and is here usually grooved by 

 the longitudinal sinus : but in a few birds it developes a bony 

 ' falciform ' ridge, which, in Buceros galeatus, e. g., bisects the 

 fore part of the prosencephalic compartment. 



The principal foramina observed in the cranium are, in the 

 epencephalic fossa, one or more minute ' precondyloid,' the large 

 foramen for the ' vagus ' and internal jugular vein, the meatus 

 auditorius internus ; in the mesencephalic fossa the ' foramen 

 ovale ' for the third and second division of the ' fifth,' the ^ carotid,' 

 which opens into the deep ' sella,' the * foramina,' which trans- 

 mit nerves to the orbit, not always distinct from the wide foramen 

 opticum ; this also being blended with its felloAv in many birds ; 

 in the prosencephalic compartment, are the rhinencephalic fora- 

 mina, which, in Apteryx and Dinornis, from the backward exten- 

 sion and interorbital position of the enormous olfactory chambers, 

 become ' rhinencephalic fossas,' distributing thereto olfactory 

 nerves by a * cribriform ' plate. 



The tympanic cavity is formed by the paroccipital, basi- and 

 ali-sphenoids, petrosal, mastoid, and tympanic. It presents the 

 stapedial canal leading to the '^ fenestra ovalis ; ' and pneiunatic 

 apertures by which the air from the Eustachian tube is conducted 

 to the pericranial diploe. The ^ petrosal ' as the osseous capsule 

 of the acoustic organ, and the ' stapes,' mth the cartilaginous 

 ' incus ' and ^ malleus,' as appendages thereof, will be noticed in 

 connection mth the sense-organs. 



The orbits are large and lateral, but encroach upon the anterior 

 wall of the cranium, the eyeballs moulding it into a pair of con- 

 cavities looking forward and usually a little downward and out- 



' xvr. vol. iv. pi. 24, fig. 4. 



