CAVITY OF THE ORBIT 



857 



of rotation at right angles to one another. By rotation of the eye on its vertical 

 axis the cornea is moved outwards (towards the temple), and inwards (towards the 

 nose): movements called respectively abduction and adduction. In upward and 

 downward movements of the cornea, the eye rotates on its liorizontal (Mjuatorial 

 axis. The other ])rincipal axis of rotation is the sagittal, which we liave i>revi()usly 

 described as corresponding to the line joining the anterior and ])osterior i)olesof the 

 globe (page 843). In rotation of the eye on its sagittal axis, therefore, the cornea 

 may be said to move as a wheel on its axle, for its centre now corresj)onds to one 

 end of the axis; in other words, this is a rotation of the cornea. Such movements 

 may, consequently, be expressed with reference to their effect on an imaginary 

 spoke of the corneal wheel — e.g. one running vertically upwards from the corneal 

 centre. Thus we may say 'rotation of the cornea outwards' when this part of the 

 wheel moves towards the outer canthus, or ' inwards ' when towards the nose. 



The only two muscles that rotate the eyeball merely on one axis are the 

 external rectus nnd the internal rectus ; the former abducting, and the latter 

 adchictiuii; the cornea. 



f'iG. 484. — View of Left Orbit feom above, showing the Ocilar Miscles. 

 (From Hirschfeld and Leveille.) 



Internal rectus muscle 



Superior oblique muscle 



Trochlea 



Levator palpebrae superioris 

 muscle, cut 



External rectus muscle 



Inferior oblique muscle 

 Superior rectus muscle 



Levator palpebrae superioris 

 muscle, cut 



The chief action of the superior rectus is to draw the cornea uj^wards, but at 

 the same time it adducts and rotates the cornea inwards. 



The inferior rectus mainly draws the cornea downwards, also adducting it and 

 rotating it outwards. 



The chief action of the superior oblique is to rotate the cornea inwards, also 

 drawing it downwards and sliglitly alKlucting it. 



Tlie inferior oblique mainly rotates the cornea outwards, also drawing it 

 upwards and slightly abducting it. 



The fasciae of the orbit. — The orbital contents are bound together and sup- 

 ported by fibrous tissues, which are connected with each other, but Avhich may 

 conveniently be regarded as belonging to three systems. These are: (1) Those 

 lining the bony walls; (2) those ensheathing the muscles; and (3) the tissue which 

 partially encapsules tlio eyol)all. 



1. The orbital periosteum, or periorbita, is closely appli id totlie I)oncs form- 

 ing the walls of the cavity, but may be stripped off with coin]>arative ease. It 

 presents openings for the passage of vessels and nerves entering and leaving tlie 

 orbit. Posteriorly tliis tissue is very firm, being joined by jirocesses of the dura 

 mater at the optic canal and sphenoidal fissure; at the optic foramen it is also 



