178 THE ANATOMY OF THE HORSE. 



the membrana nictitans before it. In the eye of a subject just dead, 

 this mechanism may readily be demonstrated by pressing the eyeball 

 backwards into the orbit. About the centre of the outer face of the 

 cartilage there will be found a cluster of reddish-yellow granules — the 

 Harderian gland. The gland secretes an unctuous material which is 

 discharged by a number of ducts that perforate the cartilage and open 

 on its ocular surface. 



The Lachrymal Apparatus comprises — the lachrymal gland with its 

 excretory ducts, the puncta lachrymalia, the lachrymal canals, the 

 lachrymal sac, and the lachrymal duct. 



The Lachrymal Gland is placed within the orbit, beneath the 

 supraorbital process of the frontal bone. The gland itself will be 

 dissected at a later stage. 



The excretory ducts of the gland discharge themselves by a number 

 of minute openings on the inner surface of the upper eyelid, close to the 

 temporal can thus. Sometimes a few of the ducts open on the lower lid 

 close to the same canthus. The watery secretion which issues from 

 them is carried over the front of the eyeball by the movements of the 

 eyelids, and at the nasal canthus it is drained away by the puncta 

 lachrymalia. 



The Puncta Lachrymalia. Each punctum is placed on the inner 

 surface of the lid near its free edge, and distant about J of an inch from 

 the caruncula. The lower punctum is generally larger and more easily 

 found than the upper. If a flexible bristle be passed into each punctum 

 it may be directed along the lachrymal canal, into the lachrymal sac. 

 Taking the bristle as a guide, each canal may then be slit open with 

 scalpel or scissors. 



The Lachrymal Canals will be found to converge towards the roof 

 of the lachrymal sac, into which they open by distinct orifices. The 

 upper canal is a little longer than the lower. 



The Lachrymal Sac is a small reservoir lodged in the fossa of the 

 same name on the orbital surface of the lachrymal bone. It receives 

 the lachrymal secretion from the lachrymal canals, and it is directly 

 continued as the lachrymal duct. 



The Lachrymal Duct {ductus ad nasum) conveys the lachrymal 

 secretion from the sac to the lower part of the nasal fossa. In the first 

 part of its course it is lodged in an osseous canal, along which it passes 

 to reach the middle meatus of the nose, where it will subsequently be 

 found. The lachrymal canals, sac, and duct have a fibrous wall with a 

 mucous lining, the epithelium being stratified and squamous in the 

 canals, but ciliated in the sac and greater part of the duct. 



The Conjunctiva. This is a mucous membrane, consisting of a 

 palpebral part lining the inner aspect of the eyelids, and an ocular 

 portion which is reflected on the front of the eyeball. The palpebral 



