4IO SENSE-ORGylNS 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- 



^ , , lae^e (a), passing from its edge to its inner 



Fig. 165. — Inner t> v -" r & t> 



Surface of Mem- angle, and the inner end of this cartilage is 

 BRANA NICTITANS, g^rroundcd, on the medial (concave) side of 



showing the support- 111 1 tt 



mg cartilage and liar- the membrane, by the large lobulated Har- 



derian glands a, sup- ^^^^^^ gla„ds (b). 

 porting cartilage; o, ^ ^ ' ^ 



iiarderian glands; c. The conjunctiva is the thin membrane 

 outer edge. 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 palpebrae 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. 



