270 VERTEBRATE ZOOLOGY 



arates the nasal capsules from each other; the cribriform plate 

 is a transverse, perforated bone, which forms the anterior wall of 

 the brain cavity and gives passage to the fibers of the olfactory 

 nerve into the nasal capsules; the labyrinths are folded bony 

 plates, which are attached to the cribriform plate on each side and 

 form the turbinals of the nasal capsules. 



The nasal cavity occupies the anterior end of the skull. It is 

 divided into a right and a left nasal cavity by the vertical mes- 

 ethmoid bone, while the hinder wall is formed by the cribriform 

 plate. The floor of the nasal cavities is formed by the paired 

 palatine bones, and its roof by the paired nasal bones. The vomer 

 is a narrow bone which lies along the ventral edge of the mes- 

 ethmoid and helps to form the nasal septum. The hinder part of 

 each cavity is divided by a horizontal ridge of the vomer into a 

 dorsal and a ventral compartment. The latter is the smaller and 

 respiratory part of the nose; the former is the olfactory part. 

 Note the external and internal nares. 



The auditory cavity of the skull has already been studied. 



Note the frontal and the sphenoidal sinus, open spaces in the 

 frontal and presphenoid bones ; they communicate with the nasal 

 cavity. 



Exercise 60. Draw a view of the section of the skull showing these 

 cavities and the boundaries of the bones forming them. 



Of the bones just studied the following belong to the cranium 

 proper, or brain case: the occipital, basisphenoid, presphenoid, 

 and ethmoid, all of which are cartilage bones and form its base ; 

 and the parietals and frontals, which are membrane bones and 

 form its roof. The special-sense capsules are three in number. 

 The auditory capsule is lodged in the temporal bone. As we have 

 seen, this bone is made up of three parts,— the petrous, tympanic, 

 and squamous portions. The petrous part, which contains the 

 inner ear and is a cartilage bone, is formed by a fusion of the 

 prootic, epiotic, and opisthotic bones (in many lower vertebrates 

 these bones are distinct and separate). The tympanic portion, 

 which contains the middle ear, is a membrane bone. The squa- 

 mosal is also a membrane bone ; it forms a part of the wall of the 



