126 VERTEBRATE ZOOLOGY 



their appearance which develop not in cartilage but in the skin 

 and in connective-tissue membranes which cover the skull ; these 

 are called membrane bones. Thus, so far as the material of which 

 it is made up is concerned, the skull is composed of cartilage, carti- 

 lage bones, and membrane bones. 



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

 paratus which supports the tongue. These structures belong to the 

 visceral skeleton. The hyoid is a thin plate of cartilage embedded 

 in the muscles of the lower jaw, with a pair of long anterior pro- 

 jections and a pair of short posterior projections. The thin plate is 

 called the body of the hyoid. The anterior projections, or anterior 

 cornua, are slender, cylindrical cartilages extending first backward 

 to the hinder end of the lower jaw and then upward to the base of 

 the cranium ; the posterior cornua are a pair of straight, bony rods 

 which extend back from the hinder end of the body of the hyoid. 



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, and a cartilage. The latter is called Meckel's cartilage ; it 

 forms the primitive mandible at the time in the development of 

 the animal when the entire skull is cartilaginous, and persists as 

 the axis of the adult mandible. Its hinder end is widened and 

 articulates with the upper jaw. 



The greater part of Meckel's cartilage, however, is not free, but 

 is insheathed by two long membrane bones,— the angular and the 

 dentary, just mentioned. The former of these bones covers the 

 ventral and medial sides of the cartilage along almost its entire 

 extent ; on the outer side the hinder part of the cartilage is still 

 exposed. The dentary bone consists of two portions,— a long, 

 flat, lateral and a small, medial portion. The former covers the 

 outer side of most of the forward half of Meckel's cartilage ; the 

 latter is much smaller and lies at the forward end of the cartilage. 

 The mandible is without teeth. 



Without disarticulating the lower jaw, carefully dissect the 

 hyoid apparatus from the floor of the mouth. 

 Exercise 34. Draw the hyoid apparatus on a large scale. 

 Exercise 35. Remove and clean the mandible and draw the dorsal 

 aspect. 



