ACTIVE ORGANS OF MASTICATION. 295 



forwards, others vertically, and both portions are inserted, 

 tendinous and fleshy, into the external surface of the 

 angle and ramus of the lower jaw as high as the coronoid 

 process. 



Function. If the anterior portions of both muscles act 

 together, the jaw is carried forwards if the posterior act, 

 it is carried backwards ; if both anterior and posterior, on 

 opposite sides, act together, the lower jaw will be power- 

 fully raised to the upper. If the superficial portion, on 

 the one side, act alone, it can throw the chin to the oppo- 

 site side. If the deep portion act by itself, it can rotate 

 the jaw to its own side. 



This muscle is one of the chief agents in mastication, as 

 it has the power of directly bringing the lower jaw to the 

 upper, and thereby dividing the food, and also of rotating 

 it, whereby its trituration is effected. 



The masseter is covered by the skin, a few fibres of the 

 platysma, orbicularis palpebrarum, and zygomatic muscles, 

 as well as by a part of the parotid gland and duct, and the 

 transverse facial vessels and nerves. 



2. Temporalis Muscle (tempora, the temples.) 



Dissection. Make an incision along the semicircular 

 ridge upon the side of the cranium, extending from the 

 external angular process of the FIG. 83. 



frontal bone, along the parie- 

 tal, back to themastoid process 

 of the temporal bone. Turn 

 this portion of skin down to 

 the zygoma, and the temporal 

 aponeurosis is brought to view. 

 This is a white, strong, and 

 shining fascia, which is at- 

 tached above to the semicir- 

 cular ridge, and below to the 

 zygoma. It covers the temporal muscle is thin above 

 where the muscle is seen through it, and thick below where 



Fio. 83 Temporal Muscle, the fascia being removed. 1 Temporal muscle, 

 2 External lateral ligament, 3 Insertion of temporal muscle. 



