856 ORGANS OF SPECIAL SENSE 



As regards length, the muscular belly of the superior rectus has the longest course, 

 and the others dmnnish in the order — internal, external, and inferior rectus. The 

 external rectus is supplied by the sixth nerve. The other three recti muscles are 

 all supplied by the third nerve. 



The levator palpebrae superioris courses along the roof of the orbit close to 

 the periosteum for the greater part of its course, partially overlajiping the superior 

 rectus; it finally descends through the orbital fat, and widens out to be inserted 

 into the root of the upper lid. It may be briefly descriljed as being inserted in two 

 distinct layers separated by a horizontal interval. The upper or anterior layer of 

 insertion is fibrous, and passes in front of the tarsus, Avhere it comes into relation 

 with fibres of the orbicularis. The lower layer consists of smooth muscle (Miiller's 

 superior palpebral muscle) and is inserted along the upper border of the tarsus. 

 The levator has also connections with the sheath of the superior rectus. These 

 different insertions of the muscle will be referred to later along Avith the description 

 of the orbital fasciae and of the upper eyelid. It gets its nervous supply from the 

 third nerve, but the smooth muscle developed in its lower layer of insertion is sup- 

 plied by the sympathetic nervous system. As its name expresses, its action is to 

 raise the U|)per lid and to support it while the eye is open. 



The superior oblique runs forward close to the inner part of the orbital roof 

 until it reaches the fossa trochlearis near the internal angular process, where it 

 becomes tendinous and passes through a fibro-cartilaginous pulley attached to the 

 fossa just named. On passing through this pulley, or trochlea, the tendon bends 



Fig. 483. — Diagrammatic Repeesentation of Origins of Ocular Muscles at the Apex 



OF THE Right Orbit. 

 (After Schwalbe, slightly altered.) 



Superior rectus 

 FOURTH SERVE 



Lachrymal ana 



Levator palpebrae superioris 



d frontal vein __\,i-_j[ ^^%\T^~ Superior oblique 



from fibrous ^^^#v, ^% OPTIC FORAMEX AXD 



1 fissure _E-® ^^^ I'/ 



Origin of external rectus from fibrous ^^fy^, V^^rT^ OPTIC FOR \ VEX A XD NER VE 



bridge over sphenoidal " -^^ 



ya.-iu-ciliary of fifth nerve Z^^ ©^^-o" Internal rectus 



SIXTH NERVE "^"^ / ^l"'^ ^\ 



/ \ N Inferior rectus 



External rectus THIRD NERVE 



at an angle of 50°, running backwards and outwards under the superior rectus to 

 its insertion into the sclerotic. It is supplied by the fourth nerve. 



The inferior oblique arises from the front of the orbit, about the junction of its 

 inner and lower walls, just external to the lower end of the lachrymal groove. It 

 runs, in a sloping direction, outwards and backwards, lying at "first between the 

 inferior rectus and the orbital floor, then between the external rectus and the globe; 

 finally it ascends slightly, to l)e inserted by a short tendon into the sclerotic at tlie 

 back of the eye. Its nervous supply is derived from the third nerve. The precise 

 manner of insertion of the different ocular muscles has been described above in our 

 Examination of the Eyeball. (For INIuscles of the Eyelids and Eyebrows, see 

 pages 429 and following. ) 



Action of the ocular muscles. — While rotating the globe so that the corneals 

 turned in different directions, the ocular muscles do not alter the position of the 

 eyeball in the orbit either laterally, vertically, or antero-posteriorly. In speaking, 

 therefore, of the eye being moved upwards, or outioards, etc. , it is the altered position 

 of the cornea or front of the eye that we mean to express; it is manifest that, if the 

 cornea moves up, the back of the eyeball must simultaneously be depressed, and 

 similarly with other movements. All the movements of the globe take i)lace by rota- 

 tion, on axes passing through the centre. Though the possible axes are numerous 

 in combined muscular action, there are three principal axes of rotation of the eye- 

 ball, and in reference to these the action of individual nniscles must be described. 

 Two of these axes are horizontal, and one vertical; they all pass through the centre 



