494 



HUMAN ANATOMY. 



.Temporal muscle 



Condyle 

 of jaw 



External 

 pterygoid 

 muscle 



246), but can now be better understood. It should be remembered that the muscles 

 of mastication are exceptionally irritable and are all supplied by the motor branch of 

 the mandibular division of the fifth nerve. When the mouth is opened very widely, 

 as in yawning, or in an effort to take an unusually large bite, the deep posterior ver- 

 tical fibres of the masseter ( which are the only ones attached to the ramus and aiding 

 m closing the mouth that do not run forward as well as upward) are carried behind 

 the centre of motion, so that their contraction tends still further to open the mouth 

 or to keep it open. Reflex contraction from overstretching is excited in the general 

 group, and the external pterygoid acting with most advantage in that position, 

 draws the condyle into the zygomatic fossa, where it is held by the masseter and 

 internal pterygoid. 



' ' Noisy movement' ' of the temporo-maxillary joint is often due to weakness of 

 the muscles of mastication, permitting the joint surfaces to fall apart as the result of 

 the slight lengthening of the ligaments produced in time by the weight of the ja-w. 



Paralysis and spasm of the facial and masticatory muscles will be considered in 

 relation to the nerves supplying them (pages 1255, 1248). 



The most frequent congenital defect of the muscles of the face is in connection 



with harelip, in which deformity 

 Fig. 508. _^^^ the portion of the orbicularis 



oris corresponding to the cleft 

 is absent. 



Dermoids are not infre- 

 quently found at the angles of 

 the orbit, in the cheeks near 

 the corner of the mouth, in the 

 naso-labial furrows, at the root 

 of the nose, and in the mid-line 

 of the chin. Reference to the 

 embryology of the face will show 

 that these .are localities in which 

 epiblastic inclusion is likely to 

 occur. 



Marked congenital asym- 

 metry of the face may occur 

 from failure of developmental 

 processes. 



Landmarks. — Just within 

 the mid-point of a line drawn 

 from the mastoid process to the 

 external occipital protuberance 

 the occipital artery can be felt as, with the great occipital nerve, it enters the scalp 

 on its way to the vertex. 



The superficial temporal artery can be felt, and often can be seen where it runs 

 over the base of the zygoma in front of the ear. Its vein and the auriculo-temporal 

 nerve are just behind it. The division of the artery into its anterior and posterior 

 branches takes place about 5 cm. (2 in.) above th^ zygoma. These branches are 

 easily palpable on the firm underlying structures, and thus afford testimony as to the 

 presence or absence of arterial degeneration. In old persons they are often tortuous 

 and plainly visible, especially the anterior branch where it crosses the anterior por- 

 tion of the temporal muscle. The region is a frequent seat of cirsoid aneurism. 



At the junction of the middle with the inner third of the supra-orbital bony 

 margin the supra-orbital notch may be felt. From this point the supra-orbital nerve 

 and artery pass almost directly upward, crossing the orbital margin. Between that 

 point and the root of the nose the frontal artery and supratrochlear nerve ascend 

 and the frontal vein descends. 



The movement of the condyle of the inferior maxilla up to the summit of the 

 eminentia articularis when the mouth is open and the external pterygoid contracts, 

 and its return into the glenoid cavity when that muscle is relaxed and the mouth is 

 closed, can plainly be felt. 



Masseter 

 muscle, partly 

 cut away 



Mandible 



Dissection showing position of dislocated jaw, condyle having 

 slipped in front of articular eminence. 



