284 TEE MUSCLES. 



arise from the inner face of the scutiform plate, pass backwards, and termi- 

 nate at the base of the concha, behind the infundibuhform cavity which that 

 cartilage forms at its root. This muscle is antagonistic to the external muscle 

 of this name, as it turns the opening of the ear outwards, and even backwards. 



8. Mastoido-auricularis. 



This name is given to a very thin fasciculus lying vertically on the inner 

 side of the cartilage, at the entrance to the ear. Arising from the margin of 

 the auditory external hiatus, and attached to the base of the concha, this little 

 muscle, in contracting, constricts the cartilaginous tube with which it is in contact. 



D. Masseteric, or Temporo-maxillary Eegion. 



This region comprises five pair-muscles for the movement of the lower jaw. 

 These are : the masseter, temporalis, pterygoideus internus, pterygoideus extenius, 

 and digastricus. 



Preparation. — 1. First study the digastricus and its stylo-maxillary portion, with the 

 internal pterygoid, in preparing the hyoid muscles as they are represented in Fig. 171. 2. 

 Expose the pterygoideus externus, by removing in this preparation the hyoid bone and its 

 dependencies, as well as the two preceding muscles. 3. 'I'o dissect the temporalis, excise the 

 external pterygoideus from its inferior bolder, an operation which exposes the orbital fasciculus 

 of the temporalis; then turn over the piece, snw off the orbital process at each end, and remove 

 the eye and auricular muscles. 4. Dissect tlie masseter, in clearing away from its external 

 surface the cuticularis and the vessels and nerves which cover it. 



1. Masseter (Fig. 168, 23). 

 Synonyms. — Zygomatico-maxillaris — Girard. 



Situation — Form — Structure. — Applied against the external face of the branch 

 of the lower jaw, the masseter is a short, wide, and very thick muscle, flattened 

 on both sides, irregularly quadrilateral, and formed of several superposed planes, 

 two of which are perfectly distinct towards the temporo-maxillary articulation, 

 by the somewhat different direction of their fibres. These are divided by a 

 considerable number of intersections, and are covered by a strong aponeurotic 

 layer, which becomes gradually thinner backwards and downwards. 



Attachments. — The fasciculi of the masseter have their fixed insertion on the 

 zygomatic crest. Their movable insertion is on the imprints which cover the 

 ugper half of the ramus of the inferi*Dr maxilla. 



Relations. — It responds, by its superficial face, to the facial portion of the 

 cervical panniculus, to the nerves of the zygomatic plexus, and several venous and 

 arterial vessels ; by its deep face, to the inferior maxillary bone, the buccinator 

 and depressor labii inferioris muscles, the superior molar glands, and two large 

 venous branches ; by its inferior border, with the parotid duct, and the glosso- 

 facial artery and vein ; by its superior and posterior border, to the parotic gland. 

 Its deep plane responds, anteriorly, with the temporo-maxillary articulation, and 

 is so intimately confounded with the temporalis, that it is impossible to define 

 the respective limits of the two muscles. 



Action. — This muscle, the special elevator of the lower jaw, plays an important 

 part in mastication. It always acts as a lever of the third class — the middle 

 line, which represents the resultant of all its constituent fibres, passing behind 

 the last molar. 



