474 



HUMAN ANATOMY. 



supplied by the ulnar nerve, the nerve-fibres using the ulnar nerve as a pathway by 

 which to reach their destination, instead of the median nerve. 



It is important, therefore, both from the morphological and clinical stand-points, 

 that not only should the nerve along which the fibres pass to reach their destination 

 be known, but also the nerve-roots by which they issue from the central nervous 

 system. 



THE BRANCHIOMERIC MUSCLES. 



The branchiomeric muscles are those skeletal muscles which are derived from 

 the mesoderm associated with the branchial arches, and are supplied by those cranial 

 nerves whose motor fibres constitute what are termed lateral motor roots. These 

 nerves are the trigeminus, facialis, and glossopharyngeo-vago-accessorius groups, 

 and the classification of the muscles may well be according to their innervation by 

 these three nerve-groups. 



I. THE TRIGEMINAL MUSCLES. 



The trigeminal muscles stand in relation primarily to the first embryonic or jaw- 

 arch, and in the adult to the structures developed in association with this, — i.e.^ to 

 the mandible and the malleus. The mandibular muscles are represented by the 

 muscles of mastication and two muscles, the mylo-hyoid and the anterior belly of 

 the digastric, which extend between the mandible and the hyoid bone, and may be 

 termed the S2ib?fiental viusc/es. Connected with the malleus is a single muscle, the 

 tensor tympani, and an additional trigeminal muscle is found in association with the 

 soft palate, the tensor palati. 



{a) THE MUSCLES OF MASTICATION. 



1. Masseter. . 3. Pterygoideus Externus. 



2. Temporalis. 4. Pterygoideus Internus. 



I. Masseter (Fig. 495). 



The masseter is a strong quadrilateral muscle composed of two portions, sep- 

 arated at their origin and posteriorly by a quantity of loose areolar tissue, but 

 united towards their insertion into the mandible. 



Attachments. — The superficial portion arises by a strong aponeurosis from 

 the anterior two-thirds of the lower border of the zygoma, while the deeper part 

 arises directly from the posterior third of the lower border and the whole of the inner 

 surface of the zygoma. The fibres of the superficial portion pass downward and 

 slightly backward to be inserted into the outer surface of the angle of the mandible, 

 while those of the deeper portion pass more directly downward and are inserted into 

 the outer surface of the ascending ramus as high as the bases of the articular and 

 coronoid processes, encroaching to a certain extent upon the insertion of the temporal 

 muscle. 



Nerve-Supply. — By the masseteric branch of the anterior portion of the man- 

 dibular division of the trigeminus. 



Action. — To raise the mandible and, by its superficial portion, to draw it for- 

 ward to a slight extent. Owing to the fibres of the muscle being directed almost 

 perpendicularly to the lever upon which it acts, the masseter works at much less 

 mechanical disadvantage than is usual, and its action is therefore exceedingly powerful. 



Relations. — A considerable portion of the masseter is subcutaneous. Poste- 

 riorly, however, the parotid gland rests upon its outer surface, and it is crossed by 

 the parotid duct, the transverse facial artery, and branches of the facial nerve. 

 Anteriorly its deep surface is separated from the buccinator muscle by a well-devel- 

 oped mass of fat, the buccal fat-pad (page 489). 



The Parotide©- Masseteric Fascia. — Covering the anterior surface of the 

 masseter is a thin layer of fascia, the tnasseteric fascia^ attached above to the zygoma 



