THE REGION OF THE ORBIT. 43 



The nerves are the optic, the third, the fourth, the 

 ophthalmic division of the fifth and the sixth, with their 

 branches, and from the lenticular ganglion are given off 

 the ciliary (short). Paralysis of the third nerve or 

 motor-oculi, causes ptosis, or a dropping of the upper 

 lid, external strabismus, dilatation, and immobility 

 of the pupil. Paralysis of the fourth nerve, or patheti- 

 cus, causes impossibility of rotation of the eyeball, and 

 diplopia; in paralysis of the sixth, or abducens, the eye- 

 ball is turned inwards. 



Relations of Parts within the Orbit. A good idea of 

 the actual relations of the contents of the orbital cavity 

 looked upon as a cone may be obtained by regarding the 

 eyeball and optic nerve as occupying very nearly its axis, 

 and the muscles, vessels, and nerves as placed superiorly, 

 inferiorly, externally, and internally to them. A needle 

 passing through the axis of the eyeball from its anterior 

 surface backwards would traverse successively (1) the 

 ocular conjunctiva; (2) the four layers of the cornea 

 viz., anterior elastic lamina, the cornea proper, the pos- 

 terior elastic lamina, the posterior epithelium; (3) the an- 

 terior chamber ; (4) the pupil ; (5) the anterior layer of the 

 capsule of the lens; (6) the lens; (7) the posterior layer 

 of the capsule of the lens; (8) the anterior portion of 

 the hyaline membrane; (9) the vitreous humor; (10) 

 the posterior portion of the hyaline membrane; (11) the 

 three layers of the retina viz. (), Jacob's membrane 

 (rods and cones); (ft) the granular layer; (^) the fibrous 

 layer; (12) the choroid; (13) the sclerotic. 



The relations of the globe and optic nerve, considered 

 as occupying the axis of the orbital cavity, successively 

 exposed by dissection, from either surface inwards, would 

 be as follows : 



