232 



SENSE OF SIGHT. 



evidence of it, when we examine how the tarsal cartilages come in con- 

 tact. Such a canal, destined for the purposes mentioned, would seem 

 superfluous. Besides the eyelashes, certain compound glands or follicles, 

 called Meibomian, are situate in the substance of the tarsal cartilages. 

 They are thirty or forty in number in the upper eyelid ; and twenty- 

 five or thirty in the lower, are in particular furrows between the tarsal 

 fibro-cartilages and the conjunctiva, and secrete a sebaceous fluid, called 

 by the French chassie, when in the dry state ; by the Germans 

 Augenbutter, ("eyebutter,") and by us, gum of the eye. It 

 serves the purposes of follicular secretions in general. 



The arrangement of the eyelids differs in different animals. In 

 several, both move; but, in others, only one; either the lower rising 

 to join the upper, or the upper descending to meet the lower. In the 

 sunfish tetraodon mola the eyelid is single and circular, with a per- 

 foration in the centre, which can be contracted or enlarged, according 

 to circumstances. In many animals there is a third eyelid, called 

 nictitating membrane, which is of a more delicate texture and more 

 largely supplied with bloodvessels ; and in some animals is transparent. 

 In birds it exists, and is well seen in the owl. It is at the inner angle 

 of the eye ; arid is capable of being drawn over the ball like a curtain 

 by two special muscles, and of thus freeing the surface of the eye from 

 extraneous substances. In man, it is only a vestige, destined to no 

 apparent use. It is called valvula or plica semilunaris. 



The eye has its proper muscles, capable of moving it in various 



directions. Their arrangement is 

 readily understood. They are six 

 in number: four recti or straight 

 muscles; and two oblique. 1. Rectus 

 superior or levator. 2. Rectus in- 

 ferior or depressor. 3. Rectus ex- 

 ternus or adductor; and 4. Rectus 

 externus or abductor. All arise 

 from the base of the orbit, around 

 the optic foramen; pass forward to 

 vanish on the sclerotica; and, ac- 

 cording to some anatomists, extend 

 over, and form a layer to, the cor- 

 nea. 



The oblique muscles are 1. 

 G-reater oblique, obliquus superior, 

 patheticus or trochlearis, which 



r . _ . . , ' . . 



arises from the inner side of the 



1. Ball of the eye and rectus externus muscle, foramen Opticum ; paSSCS forwards 

 2. Superior maxilla. 3. Third pair, distributed , , *. n. 



to all the muscles of the eye except the superior tO the internal OrDltar prOCCSS Ot 



,1 f r rmtl hnnp wViPrp it fpnrlnn 

 tn 6 J I DOne, Wnei 



J g re fl e cted OVCr a pulley Or trOchUa* 

 r , J , 



and CrOSSCS the Orbit to be inserted 



into the upper, posterior, and outer 



third pair. part Q f tne g^g Q f tne e y e . 2. 



Lesser oblique or obliquus inferior, whose fibres arise from the anterior 



110 - 



, -D . 



View of the Third, Fourth, and Sixth Pairs 



of Nerves. 



oblique and external rectus. 4. Fourth pair 

 going to the superior oblique muscle. 5. One of 

 the branches of the seventh pair. 6. Sixth pair, 

 distributed to the external rectus muscle. 7. 

 Spheno-palatine ganglion and branches. 8. Cili- 



