THE FROG 3 



On either side, five or six millimeters from the anterior end, are 

 small foramina for the exit of the eleventh nerve.* 



The Skull. In the skull notice that you can distinguish a 

 median portion, the cranium, and visceral or facial portions on 

 either side, the two being connected behind by lateral projections 

 (otic capsules) from the cranium and in front by means of nasal 

 capsules. (These last do not show well in the dried skull, as 

 the cartilage of which they are composed shrivels in drying.) 



Examine the hinder surface (base) of the skull, making out the 

 following points. In the centre the large opening (foramen 

 magnum) leading into the cranial cavity, bounded on either side 

 by rounded prominences (occipital condyles) for articulation with 

 the atlas. These condyles are borne on the exoccipital bones. 

 Obliquely in front of and lateral to each exoccipital is a pro- 

 otic bone forming the wall and roof of the otic capsule, and 

 extending from the outer end of each prootic down to the angle 

 of the jaws is a hammer-shaped squamosal bone. In front of, 

 below, and parallel to the shank of the squamosal is a pterygoid 

 bone to be studied later. (The columella auris, lying beneath and 

 outside of the prootic, is often missing from the prepared skull.) 



Draw the base of the skull, X4, naming the parts. 



The dorsal surface of the skull shows the exoccipitals, prootics, 

 and squamosals, already made out. On either side of the middle 

 line, in front of the foramen magnum, the roof of the cranium is 

 formed by a fronto-parietal bone, which extends forward to the 

 anterior margin of the orbits. Joining the fronto-parietals in 

 front is the sphenethmoid bone, the extent of which is better seen 

 from the side or from below. It is largely covered above by 

 the fronto-parietals and in front by the paired, triangular nasal 

 bones. 



In front of the nasals and forming the tip of the jaw are a 

 pair of premaxillary bones, each consisting of an alveolar process, 

 bearing teeth, and an ascending process which unites it to the 

 nasal. Most of the rest of the upper jaw is formed by the maxil- 

 lary bones, each of which joins the premaxillary in front and 

 is connected, a little farther back, with the nasal bone by a frontal 

 process. Behind, each maxillary is connected, by an overlapping 

 joint, to a slender, splint-like quadratojugal bone which extends 



* For this numbering see the section on the nervous system, infra. 



