312 MUSCLES OF THE LOWER JAW. 



don this process is enclosed as in a sheath, being continued 

 down all its forepart to near the last dens molaris. 



Use. To pull the lower jaw upwards, and press it against 

 the upper, at the same time drawing it a little backwards. 



N. B. This muscle is covered with a tendinous membrane, 

 called its aponeurosis, which arises from the bones that give 

 origin to the upper and semicircular part of the muscle ; and 

 descending over it, is inserted into all the jugum, and the 

 adjoining part of the os frontis. 



The use of this membrane is to give room for the origin of a 

 greater number of fleshy fibres, to fortify the muscle in its 

 action, and to serve as a defence to it. 



2. Masseter, 



Arises, by strong, tendinous, and fleshy fibres, which run in 

 different directions, from the superior maxillary bone, where it 

 joins the os malse, and from the inferior and anterior part of the 

 zygoma, its whole length, the external fibres slanting backwards, 

 and the internal forwards. 



Inserted into the angle of the lower jaw, and from that 

 upwards to near the top of its coronoid process. 



Use. To pull the lower to the upper jaw, and by means of 

 its oblique decussation, a little forwards and backwards. 



3. Pterygoideus Internus, 



Arises, tendinous and fleshy, from the inner and upper part of 

 the internal plate of the pterygoid process, filling all the space 

 between the two plates ; and from the pterygoid process of the 

 OS palati between these plates. 



Inserted into the angle of the lower jaw internally. 



Use. To draw the jaw upwards, and obliquely towards the 

 opposite side. 



4. Pterygoideus Externus, 

 Arises from the outer side of the external plate of the ptery- 

 goid process of the sphenoid bone, from part of the tuberosity of 

 the OS maxillare adjoining to it, and from the root of the tempo- 

 ral process of the sphenoid bone. 



