434 THE ANATO:\IY OF THE HORSE 



MUSCLES WHOSE OFFICE IT IS TO MOVE THE 

 LOWER JAW 



Temporalis is situated on the top and sides of the head. Origin — from 

 the occipital, parietal, squamous plate, and zygomatic process of the temporal 

 bones. Insertion — to the coronoid process of the lower jaw. Action — to 

 raise the lower jaw, and thus to assist in mastication, 



Masseter forms the prominence of the cheek. Origin — from the inferior 

 surface of the zygomatic ridge. Insertion — to the whole of the external 

 surface of the angle of the lower jaw. Action — to elevate the lower jaw, 

 and thus assist in mastication. 



Stylo maxillaris lies behind the lower jaw. Origin — from the styloid 

 process of the occipital bone. Insertion — to the angle of the lower jaw. 

 Action — to retract the jaw and assist in opening the mouth, 



Pterygoideus externus lies within the jaw. Origin — from the ala of 

 the sphenoid bone. Insertion — into the rough depression at the inner side 

 of the root of the condyle of the jaw. Action — to raise the jaw and draw 

 it forwards. 



Pterygoideus internus is situated below the external pterygoid, and 

 passes in a more horizontal direction. Origin — from the ala of the sphe- 

 noid bone, from the palate bone, and the tuberosity of the superior maxillary 

 bone, Irosertion — to the inner side of the angle of the jaw. Action — each 

 muscle acting separately draws the jaw towards the opposite side, and the 

 two acting alternately produce the grinding motion necessary for reducing 

 the food. 



MUSCLES OF THE EXTERNAL EAR 



Six pairs of muscles move the cartilage of the ears in all directions ; but 

 they are not of sufficient importance to require any description here. 



EXTERNAL MUSCLES OF THE EYELIDS 



Orbicularis palpebrarum is a layer of thin muscular fibre, shown at Fig, 

 GO a a. It forms a plane around the edge of the lids, extending upwards 

 and downwards, and having a tendon at the inner angle, by which it is 

 attached to the frontal and lachi'ymal bones Its action is to close the lids 

 and draw them towards tlie inner angle. 



Levator palpebr.i: superioris. — Fig. GO i is a thin slip of muscle which 

 is attached above to the aponeurotic expansion and skin of the forehead, 

 and below to the orbicularis palpebrarum. Its action is to raise the upper 

 eyelid and wrinkle the brow. 



