410 SENSE-ORGANS AND INTEGUMENT. 
broad yellowish lines. They secrete a substance which pre- 
vents the adhesion of the edges of the two lids. On the edge 
of each eyelid, three to four millimeters from the medial angle 
of the eye, is one of the openings of the lachrymal canals. 
At the medial angle of the eye is the large nictitating 
membrane (membrana nictitans), or ‘‘third eyelid,’’ corre- 
sponding to the plica semilunaris of man. 
In the cat this is large and may cover the 
whole surface of the eye. It is supported 
(Fig. 165) by a broad central strip of carti- 
Fi, 76s — nwa lage (a), passing from its edge to its inner 
SurFacE or Mem- angle, and the inner end of this cartilage is 
BRANA NicTiTASS: surrounded, on the medial (concave) side of 
showing the support- 
ing cartilage and Har- the membrane, by the large lobulated Har- 
oe ae age; 3, derian glands (4). 
Harderian glands; «¢, The conjunctiva is the thin membrane 
ta i covering the inner surface of the lids, the 
outer surface of the eyeball, and both surfaces of the nictitating 
membrane. 
The muscles of the eyelids are M. orbicularis oculi, already 
described (page 98), and M. levator palpebre superioris, 
described below. 
3. Lachrymal Apparatus.._The lachrymal gland is a 
large reddish gland, not lobulated externally, which lies on 
the surface of the eyeball just beneath the lateral angle of the 
eye. The gland is situated immediately craniad of, and in 
contact with, the zygomatic process of the frontal bone (Fig. 
154, 11, page 374). It is flat and about one centimeter in 
diameter. The ducts which pass from the gland to the eye 
are not visible to the naked eye. The fluid secreted by the 
gland collects at the medial angle of the eye and passes into 
the two openings of the lachrymal canals, one of which is 
found on the pigmented edge of each lid, three or four milli- 
meters from the medial angle of the eye. The canals pass- 
ing from these openings soon unite to form the nasolachrymal 
duct, which passes through the lachrymal bone, along the 
medial surface of the maxillary, to open into the nasal cavity 
ventrad of the ventral concha of the nose. 
