444 THE MUSCLES 



4. PTERYGOIDEUS INTER NUS 



The pterygoideus internus — named from its origin and relative position — is a 

 thick ([iiadrilateral sheet. 



Origin. — ( 1) The whole of the inner surface of the external pterygoid plate and 

 the adjacent part of the tuberosity of the palate bone. (2) A small triangular area, 

 consisting of the outer surface of the tuberosity of the palate Ijone, and a small 

 strip in front belonging to the maxilla. 



Insertion. — The lower half of the internal surface of the ramus of the man- 

 dible, including the adjacent parts of its lower and posterior borders, and extending 

 as high as the mylo-hyoid ridge and inferior dental canal. 



Structure. — Arising fleshy, the fibres run |)arallel to one another, downwards, 

 backwards, and outwards, to be inserted partly into the mandible, and partly in 

 multipenniform fashion, like those of the masseter, into the tips and sides of 

 fibrous septa, which, passing ui)Avards from the periosteum of the mandible, 

 separate the muscular bundles from one another. 



Nerve-supply. — The internal jDlerygoid branch of the motor portion of the 

 mandil)ular division of the fifth nerve, which enters the deep surface near its poste- 

 rior border. 



Action. — (1) To close the jaw. The same remarks which were made with 

 respect to the very small loss of mechanical advantage in the masseter muscle 

 apply to this muscle. (2) When closed it will draw the jaw forwards; and also 

 (3) it will help the external pterygoid in drawing the ramus of its own side towards 

 the middle line. 



Relations. — Superficially, the external pterygoid muscle, the internal lateral 

 ligament, the internal maxillary vessels, the inferior dental and lingual nerves; 

 deeply, the tensor palati and superior constrictor of the pharynx, the stylo-hyoid 

 muscle and the posterior belly of the digastric, the submaxillary gland. 



THE MUSCLES AXD FASCI.E OF THE FRONT 

 OF THE NECK 



The platysma myoides has been already described (page 426) with the muscles 

 of expression. 



The muscles of the neck Avhich lie beneath it are surrounded by a layer of deep 

 fascia, called the cervical fascia. This is a strong tubular membrane, attached 

 above to the lower l)order of the mandible, the parotid fascia, the upper part of 

 the mastoid process, and the superior nuchal line. At the back of the neck it is 

 continuous with the deep fascia which gives a thin investment to the trapezius 

 muscle. In front, it is attached to the lower l:)order of the body and great cornu of 

 the hyoid bone. Below, it ends upon the front surface of the presternum and the 

 clavicle. The deep layer of the cervical fascia separates from it just below the 

 hyoid bone, and runs downwards in close proximity to the sterno-hyoid muscles 

 and the other depressors of the hyoid bone, to the upper part of the posterior sur- 

 face of the sternum, and the posterior surface of the clavicle. Laterally, it blends 

 Avith and completes the sheath of the sterno-mastoid muscle, which is ]iartly formed 

 by the superficial layer of the cervical fascia; it also binds down the })osterior belly 

 of the omo-hyoid to the clavicle and first rib. Below its attachment to the sternum 

 and clavicle it is continued downwards in front of the trachea and great vessels at 

 the root of the neck into the superior mediastinum, and it finally joins the peri- 

 cardium. Behind, it gives an investment to the depressors of the hyoid bone. 

 Between this deep layer of the cervical fascia and the superficial layer is a small 

 space containing a part of the course of the anterior jugular vein, a lymphatic 

 gland, some fat and loose connective tissue. 



A still deeper fibrous layer, the prevertebral fascia, stretches across the neck 

 and divides the cvlindrical tub(> formed l)y the cervical fascia into a posterior and 

 anterior compartment. Its surfaces are directed forwards and liackwards. Behind, 



