ORGAN OF SIGHT IN MAMMALIA. 



281 



206 



Harderian gland and the retractor oculi co-exist, as usual, with the 

 nictitating eyelid. This is largely developed, and the conjunctiva 

 covering its free margin is stained black. Beneath the upper eyelid, 

 in the Kangaroo, there is a cartilaginous ridge having the conjunc- 

 tiva reflected over it. There are no palpebral cilia in Didelphis. 

 The Harderian gland subserves the movements of the third 

 or nictitating lid, and with the choanoid muscle, are present in 

 all quadrupeds up to the Quadrumana. In these, as in Man, the 

 third lid is reduced to a 

 small fold, fig. 206, ^, at 

 the inner canthus, within 

 and projecting a little be- 

 yond the vascular protu- 

 berance called ( caruncula 

 lacrymalis,' ib. f: the Har- 

 derian gland ceases to be 

 developed : the true lacry- 

 mal gland at the upper and 

 outer part of the orbit, fig. 

 209, k, /, is large. In fig. 

 206 the orifices of the ' tar- 

 sal ' or f meibomian ' glands 

 are shown at , CL. In Man and Quadrumana the upper of the 

 two horizontal lids is the largest and most movable, contrary to 

 the case in most lower Mammals. 

 The fibrous tissue within that fold 

 of skin is now condensed to form 

 a ( tarsal cartilage,' largest and 

 most conspicuous in the upper lid, 

 of which it forms the basis : its 

 straight and thick border consti- 

 tutes the ciliary margin. In the 

 lower lid the so-called t cartilage ' 

 is hardlv more developed than it 



- Section of eyelids showing extent of conjunc- 



1S in both lids Of quadrupeds. Ihe tive membrane and ducts of lacryrnal gland. 



The eyelids of the left side opeued. xcvni". 



207 



CX". 



meibomian follicles extend into the 

 fibrous (lower lid) or fibrocartilaginous (upper lid) tissue. The 

 muscle closing the lids is the f orbicularis palpebrarum,' fig. 29, o. 

 The upper lid is raised by a special muscle, ' levator pal- 

 pebras superioris,' which extends from the upper border of 

 the optic foramen, to the tarsal fibro-cartilage. The lower lid 

 on the relaxation of the s orbicularis' which draws it up, falls 

 down by its own elasticity : rarely in Mammals has it a proper 



