OF THE MOTOEY APPARATUS IN MAN. 131 



remain distinct throughout life in many mammals and others, 

 and form a distinctive feature of the face. They are also 

 called premaxillary (i m Fig. 77). 



tin mo c 



Fig. 77. Head of the Horse.* 



In man the germs of the lower jawbone become fused at 

 an early age ; in many mammals it is formed of two pieces, 

 united "by cartilage at the symphysis of the chin. A brief 

 inspection of the lower jawbone shows its more remark- 

 able points : its articular surface, by which it rests on the 

 glenoid cavity of the temporal ; the coronoid process project- 

 ing upwards in front of the condyle, and to which is attached 

 the powerful temporal muscle ; the alveolar edge of the jaw, 

 forming its dentar portion ; the symphysis or union of the 

 chin, and the base and angle. The masticating muscles are 

 very powerful, especially in the carnivora ; and in some ani- 

 mals the disposition of the muscles rendering the arm of the 

 lever of resistance equal to, or even shorter than that of the 

 power, adds to their strength. 



The hyoid or lingual bones (Fig. 23), placed in front of the 

 neck, representatives of another apparatus, of which in man, 

 mammals, birds, and most reptiles, we have only the rudi- 

 ments, and which only attain their full development in fishes, 



* oc, t, f, occipital, temporal, and frontal bones ; n, nasal bone ; m, 

 superior maxillary; im, intermaxillary; mi, inferior maxillary; o, orbit; 

 I, incisive teeth ; c, canine ; mo, molar. 



t Principal muscles of the face and head : o, orbicular muscles of the eye- 

 lids, intended to close the eyelids and protect the eyes ; bb, orbicular muscle 

 of the lips, intended to close them and to contribute greatly to speech, the 

 expression of the passions, &c. ; ,/, muscles of the cheeks ; m, masseter 

 muscle, intended to close the jaws by raising the lower maxillary bone; 

 t. temporal muscle having the same function; z, zygomatic arch ; c, articu- 

 lation of the lower jaw ; a, auditory n.eatus and niastoid process. 

 K2 



