A FROG 195 



brain and the organs of special sense ; the latter surrounds the 

 mouth and pharynx, forming the framework of the jaws and 

 tongue. 



We shall begin with the study of the lower jaw and the hyoid 

 apparatus, which supports the tongue. These structures belong 

 to the visceral skeleton. The hyoid is a thin plate of cartilage, 

 with a pair of long anterior and a pair of short posterior pro- 

 jections, which is embedded in the muscles of the lower jaw. 



The lower jaw, or mandible, is a paired structure which is com- 

 posed on each side of two bones called the angular and the den- 

 tary, the former of which is posterior to the latter, and a cartilage 

 which articulates with the upper jaw. 



The mandible is without teeth. 



Without disarticulating the lower jaw, carefully dissect the 

 hyoid apparatus from the floor of the mouth. 



Exercise 28. Remove and clean the mandible and the hyoid appa- 

 ratus and draw them on a large scale. 



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

 the tympanum. 



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 portion 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. 



The cranial bones fall into two distinct groups, those form- 

 ing the brain case, or cranium proper, and those forming the cap- 

 sules of the special sense organs. The primitive cartilaginous 

 cranium, which is partly replaced or covered by these bones, per- 

 sists 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 

 posterior end of the skull and surrounding the foramen magnum 



