CHAP, in.] SIGHT. 173 



of the upper eyelid, beneath the orbicularis. It is governed by a 

 branch of the third nerve ; hence injury or disease of this nerve is 

 frequently the cause of a drooping of the upper eyelid and an 

 inability to open the eye fully. 



A portion of the tendon of the levator palpebrae closely united 

 with an extension of the tendon of the superior rectus is inserted 

 into the hinder part of the upper eyelid, where the conjunctiva 

 lining it is about to be reflected over the eyeball ; and a similar 

 extension of the inferior rectus is similarly inserted into the 

 lower eyelid. Hence a contraction of the superior rectus, while 

 elevating the visual axis, at the same time raises somewhat the 

 upper eyelid ; and in like manner the inferior rectus, while de- 

 pressing the visual axis, lowers the lower eyelid. 



Between the main tendon of the levator palpebrae and the 

 tendinous slip just mentioned lies a small bundle of plain, un- 

 striated muscular fibres, which starting from the levator, ends in 

 the hind border of the tarsus ; it is sometimes spoken of as the 

 middle insertion of the levator. A similar bundle of plain mus- 

 cular fibres connects the insertion of the inferior rectus with the 

 tarsus of the lower eyelid. These two small plain unstriated 

 muscles appear to be governed by nervous filaments proceeding 

 from the cervical sympathetic, stimulation of the cervical sym- 

 pathetic leading to contraction of these muscles and so to a 

 partial opening of the eye, and section of the same nerve prevent- 

 ing their being thrown into contraction and so contributing to 

 closure of the eyelids. In some of the lower animals this closure 

 of the eye upon section and opening upon stimulation of the 

 cervical sympathetic is very distinct. In those animals which 

 possess a third eyelid this is retracted by stimulation and comes 

 forward upon section of the cervical sympathetic. 



Stimulation of the cervical sympathetic also causes some 

 protrusion and section causes recession of the whole eyeball ; this 

 is seen at times in man in disease. 



807. The conjunctiva which lines the ocular surface of the 

 eyelids and is reflected from them over the eyeball, the line along 

 which reflection takes place being spoken of as the fornix con- 

 yunctivae, consists like the skin of the body of which it is a 

 continuation, of an epithelium or epidermis resting on a dermis 

 of connective tissue. It differs from the skin in the dermis being 

 delicate and in the epidermis being thin with a tendency for the 

 constituent cells to become columnar ; hence it is sometimes spoken 

 of as a " mucous membrane." On the ocular surface of the eyelids 

 the conjunctiva is thrown into irregular ridges or imperfect and 

 fused papillae, giving rise to a satiny appearance , here the epithe- 

 lium consists of several layers of cells, the uppermost of which are 

 flattened. Over the fornix, the epithelium consists of two or three 

 layers only, the cells in the uppermost layers being cubical or 

 columnar ; over the bulb the epithelium consists also of a few 



