THE FROG 127 



Thoroughly clean the remainder of the skull, but do not remove 

 the tympanum; the process is expedited by boiling the skull a 

 short time. 



Observe the great flatness and breadth of the skull. At its hinder 

 end is the foramen magnum, the large opening through which the 

 spinal cord enters the brain cavity. The cranium, which protects 

 the brain and the special sense organs, is the narrow medial por- 

 tion of the skull. On each side of it is the large oval opening in 

 which the eye lies. The arch-shaped sides of the skull are formed 

 by the upper jaw and other portions of the visceral skeleton. 



This part of the skull, like that already examined, is composed 

 of cartilage and bone, the cartilage forming the primitive skull, and 

 the bone being either cartilage bone, which replaces the cartilage 

 in certain places, or membrane bone, which may cover it. 



The cranial bones fall into two distinct groups,— those forming 

 the brain case, or cranium proper, and those forming the capsules 

 of the special sense organs. The primitive cartilaginous cranium, 

 which is partly replaced or covered by these bones, persists 

 throughout the life of the animal in great part and appears on 

 the surface in a number of places. In skulls which have been 

 allowed to become dry this cartilage will have disappeared. 



The cranium proper contains the following bones : At the pos- 

 terior end of the skull and surrounding the foramen magnum are the 

 two exoccipital bones ; they are cartilage bones. Each exoccipital 

 bears on its hinder surface a convex articular projection, the occipi- 

 tal condyle, by which the skull articulates with the atlas. At the side 

 of each exoccipital will be seen a portion of the primitive cartilage. 



The pair of long, flat bones which form the roof of the cranium 

 and lie directly in front of the occipitals are the frontoparietal 

 bones ; they are membrane bones which lie on top of the primitive 

 cartilage. They may be raised up if the skull has not been allowed 

 to become dry and the cartilage exposed. Note the three large 

 openings present in it. The anterior ends of the frontoparietals 

 overlap a bony ring which encircles the forward end of the brain 

 case ; this is the ethmoid, a cartilage bone. 



The ventral portion of the cranium is formed by two bones, — 

 the ethmoid, just mentioned, and the parasphenoid, or parabasal. 



