THE FROG 173 



of attachment of the lower jaw to skull. Look again for 

 teeth on the jaws. 



Make out the following bones of the skull : 



a. Beginning with the front of the skull, the one at the 

 top is the premaxillary , back of this are the bones of the 

 upper jaw, the maxillary, and in the angle of meeting of 

 the maxillaries two irregular bones, the vomers. Look on 

 the ventral side of each vomer. What do you find? Are 

 these teeth of any value to the frog? 



b. Back of the vomers find the nasal bones which ex- 

 tend back to the angular, quadrilateral girdle bone or sphen- 

 ethmoid. From this the parieto-frontal bones extend to 

 the base of the skull. Notice how these bones form a bridge 

 from the maxillaries over the brain. 



c. On the base of the skull find the prootic bones leading 

 out on either side to the squamosal, a triangular hammer- 

 shaped bone which is connected with the maxillary bone 

 anteriorly by the pterygoid and posteriorly by a small bone, 

 the quadratojugal. Draw dorsal view of skull (Xi)- 



d. On the under side of the skull find the large cross- 

 shaped bone, the parasphenoid, which forms the floor of 

 the cranium. From this bone the slender palatines extend 

 to the maxillaries on either side. Note how the skull is 

 composed partly of bone and partly of cartilage. How 

 does it compare with the skull of the young frog in this 

 respect? Do you suppose the skull of the young frog has 

 proportionately as much bone as the older one? Why? 

 What is the difference between cartilage and bone? 



(4) The Spinal Column. — At the base of the skull 

 look for a large opening, the occipital foramen, through 

 which the spinal cord passes into the vertebral column. 



