390 THE VERTEBRATE SKELETON. 



olfactory nerves pass. The plane of the cribriform plate is 

 called the ethmoidal plane, and as was the case also with 

 the occipital plane, the angle that it makes with the basi- 

 cranial axis varies much in different mammals, and is of 

 importance. The olfactory fossa in which lie the olfactory 

 lobes of the brain, is partially separated from the cerebral 

 fossa, or cavity occupied by the cerebral hemispheres, by 

 ridges on the orbitosphenoids and frontals. The presphenoid 

 is connected in front with a vertical plate formed partly of bone, 

 partly of unossified cartilage; this plate, the mesethmoid 

 (fig. 72, 7), separates the two olfactory cavities which lodge 

 the olfactory organs. Its anterior end always remains un- 

 ossified, and forms the septal cartilage of the nose. 



The brain case may then, to use the words of Sir W. H. 

 Flower, be described as a tube dilated in the middle 'and com- 

 posed of three bony rings or segments, with an aperture at each 



end, and a fissure or space at the sides between each of them. 



i 

 2. THE SENSE CAPSULES. 



Each of the three special sense organs, of hearing, of sight, 

 and of smell, is in the embryo provided with a cartilaginous 

 or membranous protecting capsule ; and two of these, the 

 auditory and olfactory capsules, become afterwards more or 

 less ossified, and intimately related to the cranium proper. 



(1) Bones in relation to the Auditory capsules. 



These bones lie on each side wedged into the vacuity be- 

 tween the lateral parts of the occipital and parietal segments ; 

 they are three in number, the periotic, the tympanic and the 

 squamosal. 



The periotic is the most important of them, as it replaces 

 the cartilaginous auditory capsule of the embryo, and encloses 

 the essential organ of hearing. It commences to ossify 

 from three centres corresponding to the pro-otic, epi-otic and 

 opisthotic of lower skulls, such as those of the Turtle and 

 Crocodile. 



