MUSCLES OF THE LOWER JAW. 311 



of the inferior ascending, and decussating each other about the 

 corner of the mouth, run along the lip to join those of the oppo- 

 site side, so that the fleshy fibres appear to surround the^mouth 

 like a sphincter. 



Use. To shut the mouth, by contracting and drawing both 

 lips together, and to counteract all the muscles that assist in 

 forming it. 



There is another small muscle described by Albinus, which he 

 calls Nasalis labii superioris ; but it seems to be only some 

 fibres of the former connected to the septum nasi. 



The orbicularis, possesses a very varied and extensive action, 

 and may act as a whole or in parts. Its simplest action is to 

 close the mouth by bringing the lips together. The upper or 

 lower labial fibres may act separately, as well as those at the 

 commissures of the lips, by which they are enabled in turn, to 

 antagonize the different muscles, which are attached around. By 

 a very strong contraction of the labial and commissural fibres, 

 the lips are thrown forwards in a circular projection, as in whist- 

 ling. By the contraction of the inner labial fibres, they are 

 drawn inwards upon the teeth. 



Muscles of the Lower Jaw. 



The lower jaw has four pair of muscles for its elevation or 

 lateral motions, namely, two, which are seen on the side of the 

 face, and two concealed by the angle of the jaw. 



1. Temporalis, 



Arises, fleshy, from a semicircular ridge of the lower and 

 lateral part of the parietal bone, from all the pars squamosa of 

 the temporal bone, from the external angular process of the os 

 frontis, from the temporal process of the sphenoid bone, and 

 from an aponeurosis which covers it ; from these different origins 

 the fibres descend like radii towards the jugum, under which 

 they pass ; and are, 



Inserted, by a strong tendon, into the upper part of the coro- 

 noid process of the lower jaw ; in the duplicature of which ten- 



