ACCESSORY ORGANS OP VISION. 



231 



which occupies the whole of the eyelids, passing from the edge of the 

 orbit to the tarsal margin, and seeming intended to limit the motion 

 of the eyelids, when they approximate each other. The last layer, 

 and that which forms the posterior surface of the eyelids, is a fine, 

 delicate, transparent, mucous membrane, called tunica conjunctiva or 

 tunica adnata; so named because it joins the eyelids to the globe of 

 the eye. It lines, in fact, the eyelids, and is reflected over the ball ; 

 but it has been a matter of contention whether it passes over the trans- 

 parent cornea. The generality of anatomists say it does; M. Ribes, 1 

 however, maintains the opinion, that it extends only as far as the cir- 

 cumference of the cornea, and that the cornea itself is covered by a 

 proper membrane. Physiologically, this dispute is of no moment. At 

 its outer surface, a humour is constantly exhaled, which keeps it moist, 

 and facilitates the motions of the eyelids over the eyeball. Its loose 

 state also favours these motions. 



Both eyelids are kept tense by the aid of a fibro-cartilage, situate 

 along the edge of each, and called tarsus. That of the upper is much 

 more extensive than that of the lower; and both seem as if cut obliquely 

 at the expense of their inner surface ; so that, in the opinion of most 

 anatomists, when the eyelids are brought together, a triangular canal 



Fig. 108. 



Fig. 109. 



Muscles of the Eyeball. 



1. A small fragment of the sphenoid bone 

 around entrance of optic nerve into orbit. 2. 

 Optic nerve. 3. Globe of eye. 4. Levator 

 palpebrae muscle. 5. Superior oblique muscle. 

 6. Its cartilaginous pulley. 7. Its reflected 

 tendon. 8. Inferior oblique muscle ; the small 

 square knob at its commencement is a piece of 

 its bony origin broken off. 9. Superior rectus. 

 10. Internal rectus almost concealed by optic 

 nerve. 11. Part of external rectus, showing its 

 two heads of origin. 12. Extremity of external 

 rectus at its insertion ; the intermediate portion 

 of muscle having been removed. 13. Inferior 

 rectus. 14. Tunica albuginea formed by ex- 

 pansion of tendons of four recti. 



Meibomian Glands seen from the Inner or Ocular 

 Surface of the Eyelids, with the Lachrymal 

 Gland the Right Side. 



a. Palpebral conjunctiva. 1. Lachrymal gland. 

 2. Openings of lachrymal ducts. 3. Lachrymal 

 puncta. 6. Meibomian glands. 



is formed between them and the ball of the eye, which has been con- 

 ceived useful in conducting the tears towards the lachrymal puncta. 

 M. Magendie 2 denies that any such canal exists ; and there seems little 



1 Memoires de la Societe Medicale d'Emulation, vol. vii., Paris, 1817. 



2 Precis Elementaire, i. 52. 



