THE FACIAL MUSCLES. 479 



quent variation, and the anterior bellies of opposite sides may be united by the more or less 

 complete conversion of the fascia which typically passes between them into muscular tissue. 

 An independent muscle extending between the body of the hyoid and the symphysis of the 

 mandible, and termed the inetito-hyoid, occasionally is found running alongside of the medial 

 border of the anterior belly, and is to be regarded as a separated portion of that muscle. 



As regards the posterior belly, it may take its origin from any part of the mastoid groove 

 or even from the outer portion of the superior nuchal line, and occasionally it fuses completely 

 with the stylo-hyoid. In certain cases in which there is a failure of the anterior belly to differ- 

 entiate from the mylo-hyoid, the posterior belly is inserted into the angle of the mandible 

 instead of into the hyoid bone, — a condition recalling the arrangement typical in the majority 

 of the mammalia, in which the posterior belly of the digastric is represented by a depressor 

 majidibiilce . 



(r) THE TRIGEMINAL PALATAL MUSCLE. 

 I. Tensor Palati (Fig. 509). 



Attachments. — -The tensor palati (tensor veil palatini) takes its origin from 

 the scaphoid fossa and spine of the sphenoid and from the outer surface of the car- 

 tilaginous portion of the Eustachian tube. It descends along the outer surface of the 

 internal pterygoid plate, and, becoming tendinous, bends at right angles around the 

 hamulus and is continued inward to be inserted into the posterior border of the palate 

 bone and into the aponeurosis of the soft palate. 



Nerve-Supply. — By fibres from the mandibular division of the trigeminus, 

 which traverse the otic ganglion. 



Action. — It tends to draw the soft palate to one side. The two muscles acting 

 together will stretch the soft palate. 



{d) THE TRIGEMINAL TYMPANIC MUSCLE. 



I. Tensor Tympani (Fig. 1252). 



Attachments. — The tensor tympani is a small bipenniform muscle which lies 

 in a bony canal situated above the Eustachian tube. Its fibres take their origin from 

 the cartilaginous portion of the Eustachian tube, the adjacent portions of the great wing 

 of the sphenoid, and also to a certain extent from the walls of the bony canal. The 

 tympanic end of the Eustachian tube is separated from the opening of the canal for 

 the tensor by a bony ridge, the processus cochleariformis, over which the tendon of 

 the tensor bends almost at right angles and passes outward across the tympanic 

 cavity to be inserted into the manubrium mallei near its attachment to the head of the 

 bone. 



Nerve-Supply. — By fibres from the mandibular division of the trigeminus, 

 which traverse the otic ganglion. 



Action. — The muscle draws the handle of the malleus inward and so tenses 

 the membrana tympani. 



II. THE FACIAL MUSCLES. 



The muscles supplied by the facial nerve are readily divisible into two groups. 

 Primarily this musculature is associated with the second branchial or hyoid arch, 

 represented in the adult by the lesser cornu of the hyoid bone, the stylo-hyoid liga- 

 ment, styloid process, and stapes, and a small group of muscles — the stylo-hyoid, the 

 posterior belly of the digastric, and the stapedius — are still found in relation to these 

 structures. From the surface of the mass from which these muscles differentiate 

 there is separated at an early stage a layer which gradually increases in extent and 

 eventually covers all the neck and head in a cowl, as it were, its progress from the 

 hyoid arch being followed by a branch of the facial nerve, which eventually, with the 

 growth of the muscle, mcreases to such an extent as to appear to be the main stem 

 of the nerve. From the muscular sheet numerous superficial muscles of the head 

 and neck develop, and the entire group so formed may be termed, from one of its 

 principal members, the platysma group, the group retaining the primary relationships 

 forming the hyoidean group. 



