ii8 THE SURGICAL ANATOMY OF THE HORSE 



The puncta are the openings ot the lachrymal canals which 

 convey the secretion ot" the gland to the lachrymal sac, after the 

 secretion has performed its functions on the eyeball. They are 

 slightly larger than the excretory ducts of the gland, and like the latter 

 they should be carefully located, as mechanical blocking of them, 

 which simply requires restoration of their patency, may lead one astray 

 by suggesting some more serious affection on account of the great 

 amount of lachrymation which is in evidence. 



The Membrana Nictitans. — At the inner canthus is also seen the 

 flexible piece of elastic cartilage called the membrana nictitans, commonly 

 known as the " haw," and sometimes referred to as the third eyelid. 



Although normally quite visible, the membrana nictitans only 

 projects to a slight extent across the front of the eyeball. It passes to 

 the inner side of the latter, and its posterior edge is connected with a 

 pad of fat placed at the back of the orbit. 



From the arrangement of the muscles of the eyeball and the 

 disposition of the cartilage, it will be seen how in tetanus, when the 

 retractor oculi muscle becomes contracted, the eyeball is drawn back- 

 wards into the orbit and presses on the pad of fat mentioned. The 

 fat is thus pushed forwards along the inner aspect of the eyeball, 

 thrusting the membrana nictitans before it. An almost completely 

 exposed membrana nictitans is therefore one of the diagnostic 

 symptoms of tetanus. In this disease it is unnecessary to treat the 

 membrana nictitans. But other causes may lead to its being exposed 

 to an abnormal extent, such as disease or defective conformation of 

 the cartilage itself 



When so exposed the appearance presented is most unsightly, and 

 is best remedied by excision of the cartilage. 



Excision of the Membrana Nictitans. — For the performance of this 

 operation local anaesthesia is sufficiently effective, and it is unnecessary 

 therefore to place the animal under a general anesthetic. Neither is 

 it necessary to cast the patient. 



