788 



ORBIT. 



The malar branch passes on to the inferior 

 external angle of the orbit, where it sometimes 

 communicates with the lachrymal nerve ; it 

 enters one or more canals in the malar bone, 

 and appears on the facial surface, supplying 

 the orbicularis and the integuments, and com- 

 municating with the portio dura. 



In addition' to the structures ordinarily 

 described as existing within the orbit, Mr. 

 OTerrall has described* a fibrous structure, to 

 which he gives the name of " tunica vaginalis," 

 and which invests the globe of the eye, sepa- 

 rating it from the muscles and fat of the orbit. 

 In order to expose the outer surface of this 

 structure, a vertical incision must be made 

 through the integument of the upper lid ; after 

 removing carefully the orbicularis and a fascia, 

 between the two layers of which the tendon of 

 the levator palpebrae is inserted, the part 

 next in order is the tarsal cartilage. Tracing 

 this upwards and backwards its thin margin is 

 found to be continuous with a fibrous lamina, 

 which passes back into the orbit and separates 

 the globe of the eye from the superior rectus 

 muscle, but presenting a well-defined opening, 

 through which the tendon of the muscle passes 

 as over a pulley, to be inserted into the scle- 

 rotic coat. In order to examine the ocular sur- 

 face of this membrane, Mr. OTerrall advises 

 a vertical division of both palpebrae, then an 

 incision through the conjunctiva at the angle 

 of reflection from the eyelid to the ball of the 

 eye. 



The incision being made and the edges of 

 the divided membrane separated, we expose 

 the ocular surface of " a distinct tunic of a 

 yellowish white colour and fibrous consistence, 

 continuous in front with the posterior margin 

 of the tarsal cartilage, and extending back- 

 wards to the bottom or apex of the orbit, where 

 its consistence becomes less strongly marked." 

 This ocular surface is smooth where the eye 

 glides over it in its movements, and is con- 

 nected to the surface of the globe by fine cel- 

 lular tissue. The muscular substance of the 

 recti muscles is on the outside of this tunic 

 and invisible through it; but about half an 

 inch posterior to its anterior margin are six 

 well-defined openings through which the ten- 

 dons of the muscles emerge in passing to their 

 insertion in the sclerotic coat. Mr. OTer- 

 rall was induced to look for this structure in 

 consequence of meeting with cases in which 

 the globe of the eye and the conjunctiva were 

 protruded in a manner not satisfactorily ex- 

 plained by reference to any previously described 

 structure. He believes these to have been cases 

 of inflammation of this tunic, with effusion 

 between its deep surface and the globe of the 

 eye. 



Mr. OTerrall believes that " the uses of 

 this tunic are to present a smooth surface, faci- 

 litating the movements of the eye; and by its 

 density and tension, to protect it from the pres- 

 sure incidental to the swelling of its muscles 

 during their action. That the openings in this 



tunic perform the office of pulleys, giving a 

 proper direction to the force exerted by the 

 muscles, securing the motions of rotation, and 

 opposing those of retraction, which would other- 

 wise predominate." 



Action of the muscles. The action of the 

 levator putpebrce muscle is to raise the upptr 

 lid, and thus to expose the anterior part of the 

 eye-ball. In this action it is an associate of the 

 frontal portion of the occipito-frontalis, and an 

 antagonist of the orbicularis palpebrarum. Sir 

 C. Bell* affirmed that the action of the levator 

 palpebrae is not simply that of raising the upper 

 lid, but that the swelling and tension of the 

 muscle during its action have the effect of 

 pushing forwards the eye-ball, thus causing the 

 lower lid to slide off the convex surface of the 

 eye, and to be depressed whilst the upper lid 

 is elevated. There is no proof of any such 

 action of the levator palpebrse, and it seems 

 improbable that it should exert any such in- 

 fluence, separated as it is from the eye-ball by 

 the superior rectus, by a considerable quantity 

 of fat and by the " tunica vaginalis." The re- 

 sult of paralysis of this muscle is a dropping of 

 the upper hd, to which the term ptosis is applied. 

 It is evident from an examination of the 

 origin, course, and insertion of the recti muscles, 

 that each of them acting singly is capable of 

 making the eye-ball revolve in its own direc- 

 tion ; the superior rectus directs the cornea 

 upwards; the inferior rectus antagonises the 

 superior and directs the cornea downwards; 

 the external rectus directs it outwards and is 

 antagonised by the internal, which draws the 

 cornea inwards towards the nose. It is also 

 evident that the action of any two contiguous 

 recti muscles will give the cornea a direction 

 intermediate between that which it would as- 

 sume from the action of each of them singly; 

 the superior and internal recti acting together 

 will direct the cornea upwards and inwards, 

 while the inferior and external will direct it 

 downwards and outwards ; so that the cornea 

 may be made to assume any intermediate posi- 

 tion by the action of the recti muscles alone. 



The successive action of all the recti mus- 

 cles would produce a movement of the eye- 

 ball analogous to circumduction of a limb ; 

 and as the circumduction of a limb is a move- 

 ment altogether distinct from rotation, so is 

 this circumduction of the eye-ball entirely dis- 

 tinct from any rotation upon its antero-posterior 

 axis. In circumduction the centre of the 

 cornea describes a circle, whereas in rotation 

 this point remains fixed, forming the anterior 

 extremity of an imaginary axis, round which 

 the circumference of the cornea revolves. It 

 is of much importance to have a definite 

 notion of each of these movements, as we shall 

 thus avoid one source of confusion in con- 

 sidering the action of the straight and oblique 

 muscles of the eye. Having thus defined 

 rotation, no argument is necessary to prove that 

 the recti muscles are incapable of producing 

 such a movement ; a glance at their direction 



* Dublin Journal of Medical Science, July, * The Nervous System of the Human Body, by 

 1841 - Sir C. Bell. 



