146 VERTEBRATE ZOOLOGY 



Exercise 21. Draw on a scale of 3 or 4 the anterior end of the atlas, 

 a side view of the axis and atlas, and the anterior end of the fourth 

 vertebra. 



Study the caudal vertebrae. Note the rudimentary ribs on the 

 anterior ones. 



The Skull. Disarticulate the lower jaw and remove it together 

 with the hyoid apparatus, being careful not to injure the latter. 

 The skull is made up of two regions, the cranium and the visceral 

 skeleton, the former constituting the brain case together with the 

 special-sense capsules, and the latter being made up of the bones 

 and cartilages of the upper jaw, the suspensorium of the lower 

 jaw, the lower jaw, and the hyoid apparatus. 



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

 apparatus, which supports the tongue. The lower jaw, or mandible, 

 is a V-shaped structure composed of five bones and a cartilage on 

 each side. The bones are very firmly knit together, and a pro- 

 longed boiling may be necessary to make the sutures visible. Be- 

 fore this is done, however, observe the shape of the mandible (the 

 cutting edge of which is covered by horn), the prominent coronary 

 process on the upper surface of each side, and the articular surface 

 at the hinder end of each side. Note also on the inner surface of 

 each side a long groove in which lies a rod of cartilage. This is 

 Meckel's cartilage. 



The bones in the mandible are the following : the dentary, which 

 is fused with its fellow of the opposite side and forms the front 

 part of the jaw and most of the outer and lower surfaces; the 

 coronoid, which forms the coronary process and serves as the prin- 

 cipal surface of attachment for the strong jaw muscles ; the sple- 

 nial, which forms the greater part of the inner surface from the 

 coronary process to the hinder end ; the angular, which is a thin 

 strip beneath the splenial ; the supra-angular, which is a small 

 plate above the angular at the hinder end ; and the articular, which 

 forms the greater part of the hinder end and is an ossification 

 in the hinder end of Meckel's cartilage ; the others are all mem- 

 brane bones. 



Exercise 22. Draw the lower jaw, showing the outlines of these bones. 



