THE RABBIT 327 



Identify the bones forming the orbit. The dorsal portion of it 

 is formed by the frontal bone, of which the supra-orbital process is 

 a part. The anterior wall of the orbit is formed by the lachrymal 

 bone, which is but loosely joined with the abutting frontal and 

 maxillary bones and easily falls out and is lost. At its ventral side 

 is the opening of the nasolachrymal canal, through which the tear 

 duct passes to the nasal cavity. Note the protrusion of the three 

 grinders which push their way into the orbit. 



Exercise 50. Draw a view of the lateral aspect of the skull, with out- 

 lines of the bones which appear ; carefully label all. 

 Exercise 51. Draw a similar view of the ventral aspect. 



With a fine saw divide the skull into two slightly unequal halves 

 by a longitudinal dorsoventral incision just at one side of the 

 median line, and study its inner surface. The interior of the skull 

 is divided into three cavities,— the cranial, the auditory, and the 

 nasal cavities, which contain the brain and the auditory and olfac- 

 tory organs respectively. 



The cranial cavity, in turn, is divided into three partially sep- 

 arated spaces,— the anterior, middle, and posterior fossae. The an- 

 terior, or olfactory, fossa, the smallest of these, is a small space 

 occupying the anterior end of the cranial cavity, and bounded in 

 front, and separated from the nasal cavity, by the perpendicular 

 cribriform plate of the mesethmoid bone. This plate occupies a 

 transverse position at the forward end of the cranial cavity and is 

 perforated by a number of holes for the passage of the olfactory 

 nerves to the nasal cavities. 



The middle, or cerebral, fossa is the largest of the three and 

 occupies the entire center of the cranial cavity. In its floor is the 

 large optic foramen, and posterior to this the sella turcica, in which 

 the hypophysis is lodged. 



The posterior, or cerebellar, fossa is separated from the middle 

 fossa by a wide perpendicular ridge on the side of the cavity called 

 the tentorium. Posterior to this ridge is the inner, or petrous, por- 

 tion of the temporal bone, which contains the inner ear and in 

 which are two large depressions. The upper and larger of these 

 contains in life the flocculus of the cerebellum and is called the 



