SUBMAXILLARY MUSCLES. 183 



that part of the external surface of the great wing of the sphenoid bone 

 which looks downwards, and to the ridge which separates that surface from 

 the temporal fossa ; while the inferior, which is larger, is attached to the 

 outer surface of the external pterygoid plate, and to the tuberosities of the 

 palate and upper maxillary bones. The fibres from both heads pass back- 

 wards, and being mixed with tendon, converge to be inserted into the fore 

 part of the neck of the condyle of the lower jaw, and into the inter-articular 

 fibro-cartilage of the temporo-maxillary articulation. 



The internal maxillary artery is placed on the outer surface of this muscle, passing 

 thence between the heads of origin ; while the buccal nerve issues from between those 

 heads. The deep surface rests against the upper part of the internal pterygoid muscle, 

 whose direction it crosses, also the internal lateral ligament of the lower jaw, the 

 inferior maxillary nerve, and the middle meningeal artery. The upper border is in 

 contact with the great wing of the sphenoid bone, and is crossed by the deep 

 temporal nerve and arteries. 



ACTIONS. The masseter, temporal, and internal pterygoid muscles are elevators of 

 the lower jaw, and generally act in concert, bringing the lower into contact with 

 the upper teeth. The opposite movement of depressing the jaw, not being opposed 

 by any resisting obstacle, requires less force, and is effected by muscles of much 

 smaller size, the principal of which is the digastric muscle hereafter described. 

 The external pterygoid muscle, having the great body of its fibres nearly horizontal, 

 draws forwards the condyle of the jaw, and, when the muscles of both sides act 

 together, the lower jaw is protracted so as to make the inferior incisor teeth 

 project beyond the upper incisors ; but their more usual mode of action is alternately 

 on the two sides, as in the grinding movement of the molar teeth, in which a variety 

 of muscular actions are combined. The external pterygoid muscles also, though 

 chiefly horizontal movers of the jaw, likewise contract in opening the mouth, the 

 condyles of the jaw being drawn forward in that movement. The masseter and 

 internal pterygoid muscles assist in protracting the jaw; the temporal alone is a 

 retractor. The two pterygoid muscles of one side, in advancing one condyle of 

 the jaw, necessarily throw the teeth towards the opposite side. 



SUBMAXILLARY MUSCLES. 

 A. MUSCLES CONNECTING THE HYOID BONE WITH THE SKULL. 



The digastric muscle, extending from the temporal bone to the lower jaw, 

 consists of two elongated muscular bellies united by an intervening rounded 

 tendon, which is connected with the hyoid bone. The posterior belly, 

 which is longer than the anterior, arises from the digastric groove of the 

 temporal bone, and tapers downwards, forwards, and inwards : the anterior 

 is attached to a rough depression situated on the lower border of the 

 jaw-bone, close to the symphysis ; it is less tapering than the posterior 

 belly, and is directed downwards and backwards. The intervening tendon 

 is connected with the body and great cornu of the os hyoides by aponeurotic 

 fibres at right angles to its own, and by the fleshy fibres of the stylo-hyoid 

 muscle, through which the tendon passes. 



The anterior belly, placed immediately beneath the deep cervical fascia, rests on 

 the mylo-hyoid muscle ; it is connected with its fellow of the opposite side by dense 

 fascia, and occasionally is united by muscular fibres to it or to the mylo-hyoid muscle. 

 The posterior belly is covered by the mastoid process and the muscles arising from 

 that bone, and crosses both carotid arteries and the jugular vein. 



