54 THE FROG 



The exoccipitals are a pair of bones which together bound 

 the foramen magnum or posterior opening of the cranium, 

 which is continuous with the neural canal of the spinal 

 column. On their posterior sur faces they bear the occipital 

 condyles, two oval convex processes which articulate with 

 the first vertebra or atlas. Near each occipital condyle is a 

 foramen by which the vagus nerve leaves the skull. 



The prootics are two bones forming the bony part of the 

 auditory capsules and lying laterally and anteriorly to the 

 exoccipitals. Each forms part of the orbit of the eye, and 

 on its antero-ventral face bears a large notch through which 

 pass the fifth, sixth and seventh cranial nerves. 



Ventral and lateral to the prootic is the large tympanic 

 cavity (the middle ear of higher vertebrates), which in the 

 fresh skull is covered laterally by the tympanic membrane 

 or drum membrane of the ear. At the bottom of the tym- 

 panic cavity is an opening, the fenestra ovalis, which leads 

 into the cavity of the auditory capsule; in the fresh skull the 

 fenestra ovalis is closed by a minute disc of cartilage called 

 the operculum, which in turn is joined with the tympanic 

 membrane by a small bony and cartilaginous rod called the 

 columella. Like the ossicles of the mammalian ear, these 

 two small structures serve to convey sound waves from the 

 tympanic membrane to the inner ear. In prepared skulls the 

 operculum and columella are often missing. 



The sphenethmoid or girdle-bone is a bony tube en- 

 circling the anterior end of the cranial cavity. In a surface 

 view of the entire cranium this bone is partly concealed by 

 membrane bones. 



(2) Membrane bones of the cranium. 



The fronto-parietals are two long flat bones meeting in a 



