2r3 MUSCLES OF THE HEAD AND NECK. 



superior and internal recti with the inferior oblique ; and in the downward and 

 inward direction the inferior and internal recti with the superior oblique. And 

 the same is tnie of the upward and outward and do'miward and outward move- 

 ments of dii'ection ; for in all these movements the action of the oblique muscles 

 is necessary to casntrol or supplement the rotatory tendency of the recti miiscles ; 

 and in the consentaneous movements of the two eyes the whole six muscles 

 must co-operate in both eyes to produce that perfect agTeement in their move- 

 ments of direction and convergence which is required for perfect vision. 



It is unnecessary here to enter into the detail of the modifications of these 

 actions of the muscles which must accompany changes in the various consenta- 

 neous movements of the eyes, as, for example, in the convergence which is asso- 

 ciated Avith tlie adjiLstment of the eyes to near and distant vision. (Consult 

 G. Johnston in article " Orbit," of Cyclopa3d. of Anat. and Physiol., Jacob in 

 Dublin Med. Press. 1811 ; John S. Wells, in the Ophthalmic Hosp. PiCports, 

 vol. ii. 18y'J-(jO ; Von Graefe, in Archiv fiir Ophthalmologic, vol. i. p. 1 ; and 

 other works of that author, and of Alfred Graefe, and other Oi^hthalmologists, 

 especially those on Strabismus. x\lso Helmholtz in liis Physiological Optics.) 



Pascise of the Orbit. —The space witliin the orbit which is not occupied by 

 the eyeball and its muscles, or other parts belonging to it. is completely filled 

 with soft fat and delicate yielding connective tissue. In various places this last 

 is condensed into layers of slender fascia3 of various degrees of strength. One 

 layer of this stnicture fonning the capsule of Tenon, sui-rotinds the eyeball on 

 every side except in front, where it is reflected on the inside of the conjunctiva 

 and eyelids, and this layer is so loosely connected with the suiTOunding jiarts 

 that it seems to serve all the purposes of a synovial membrane in the ball and 

 socket-like disjiosition of the globe of the eye in its jjadding. The various 

 muscles pierce this capsule at a short distance from the place of their insertion 

 into the sclerotic. From the capside of Tenon, septa of fascial connective tissue 

 spread outwards mainly in the direction of the recti muscles, all of whicli 

 receive delicate investments fi-om the structure ; a layer also spreads between 

 the muscles and the periosteal lining of the orbit. In two places these septa of 

 fascia are of greater strength than elsewhere, viz., on the inside towards the 

 lachrymal sac and eyelids, and on the outside towards the margin of the orbit, where 

 it is imited with the external ligament of the palpebra; and the periosteum. 



In several parts of these fascial structures, more especially in those last men- 

 tioned, plain or unstriped muscular fibres have been detected, and these, together* 

 with the elastic connective tissue are conceived to act in restoring the position of 

 parts after the action of the voluntary muscles is over. These involuntary 

 muscles are under the influence of the cer\dcal sympathetic neiwes. (See Henle's 

 Anatomied. Mensch., vol. ii., and Merckel in the Handbuch derGesammt. Augen- 

 heilkimde, 1st part, 1874; H. Mixller, Sitzungsb. d. Wurtzburg. Gesellsch, 1858, 

 and Tiuner, Xat. Hist. Hey., 1862 ; Cruveilhier, Ti-aite d'Anatomie, &c.) 



MUSCLES OF MASTICATION. 



The masseter, temporal, and two pterygoid muscles form -a group of 

 muscles of mastication, which may be properly considered together. 



The masseteric fascia is a continuation uinvards of the deep fascia 

 of the neck over the masseter muscle. It is firmly bound down to tlie 

 outer surface of the muscle, and is attached superiorly to the zygoma. 

 Posteriorly it closely invests the parotid gland {fascia 'parotidca) ; it 

 likewise sends upwards a process on the posterior and deep surfoces 

 of the parotid gland ; and a strong band of this process, the i^li/lo- 

 maxillarij ligament, extending from the angle of the jaw to the styloid 

 process, separates the parotid and submaxillary glands. 



The masseter is a thick quadrate muscle, whose fibi'es form two 

 portions differing in size and direction. The svperficial part, obliquely 

 Ibursided in form, arises from the lower border of the malar bone in 

 the anterior two-thirds of the zygomatic arch, chiefly by thick tendons 



