MUSCLES OF THE HEAD. 223 



It is an energetic elevator of the lower lip. 



To dissect this muscle, the directions given for the preparation of the anterior medius 

 will suffice. 



B. Masseteric or Temporo-maxillary Region. 



This pair region comprises five muscles for the movement of the lower 

 jaw. These are : the masseter, temporal, interned pterygoid, external pterygoid, 

 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. 111. 

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

 its dependencies, as well as the two preceding muscles. 3. To dissect the temporalis, 

 excise the external pterygoideus from its inferior border, an operation which exposes the 

 orbital fasciculus of the temporalis ; then turn over the piece, saw off the orbital process 

 at each end, and remove the eye and auricular muscles. 4. Dissect the masseter in 

 clearing away from its external surface the cuticularis and the vessels and nerves which 

 cover it. 



1. Masseter. (Fig. 110, 23.) 

 Synonyms. Zygomatico-maxillaris Girard. ^The zygomatico maxillaris of Leyh.) 



Situation Form Structure. Applied against the external face 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 articula- 

 tion, 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 down- 

 ' wards. 



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

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

 cover the upper half of the inferior maxillary branch. 



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

 cuticularis colli, to the nerves of the zygomatic plexus, and several venous 

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

 alveolo-labialis and maxillo-labialis muscles, the superior molar glands, and 

 two large venous branches ; by its inferior border, with the parotid canal, 

 and the glosso-facial artery and vein ; by its superior and posterior border, 

 to the parotid 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. 



2. Temporal or Crotapliitic Muscle. 

 Synonyms. Temporo-maxillaris Girard. (The temporo-maxillaris of Leyh.) 



Situation Form Structure. Situated in the temporal fossa, to which 

 it is moulded, and which it fills, this muscle is flattened from above to 

 below, divided by strong tendinous intersections, and covered by a nacrous 

 aponeurotic layer. 



Attachments. It takes its origin : 1, In the temporal fossa and on the 



