17-i THE ANATOMY OF THE HORSE. 



close to the same canthus. The watery secretiou 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 Lachryjialia. Each punctum is placed on the inner 

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

 the caruucula. 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 surfoce 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 luiswn) conveys the lachrymal secre- 

 tion 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 

 portion, including that covering the caruncula and membrana nictitans, 

 consists of a stratified epithelium and a papillated layer of vascular 

 subepithelial connective-tissue containing small mucous glands. The 

 ocular portion where it covers the sclerotic resembles the preceding in 

 structure, but is thinner, nonpapillated, and less vascular; wher^it 

 passes over the cornea, it consists of the epithelium only, which i^ 

 generally enumerated as one of the constituent layers of the cornea 

 itself, being tenned its antenor e2ntheliu7)i. Through the puncta 

 lachrymalia, the conjunctival epithelium is continuous with that lining 

 the lachi-ymal canals ; and at the free margin of the lids it is continuous 

 with the epidermis. 



Structure of the Eyelids. — Each lid is composed of the following 

 parts : — a layer of skin outwardly, the palpebral conjunctiva inwardly, 

 a portion of the orbicularis palpebrarum muscle, the palpebral tendon, 

 the tarsal cartilage, vessels, and nerves. 



Besides these, there is found in the upper lid the tendon of a special 



