398 THE ORGANS OF SPECIAL SENSE 



The lashes (cilia) are short, thick hairs forming a 

 double row on the free margin of each lid. Above 

 they are longer and more numerous. They protect 

 the eyes from dust, etc. 



The conjunctiva is the mucous membrane of the 

 eye. The palpebral portion is very thick and vascular, 

 and forms at the inner canthus a fold known as the 

 plica semilunaris. The ocular portion is loosely 

 connected to the sclerotic coat, but over the cornea 

 consists only of the conjunctival epithelium. 



The Lacrymal Apparatus. This includes the gland, 

 the two canals, the sac, and the nasal duct. 



The gland is about the size and shape of a small 

 almond, and lies in a depression in the orbital plate 

 of the frontal bone just inside the external angular 

 process. Its ducts, ten or more in number, run beneath 

 the conjunctiva and open separately at the outer part 

 of the fornix. 



The lacrymal canals commence by small orifices, 

 the puncta, on the margin of each lid, and empty 

 close together into the sac. 



The lacrymal sac is the upper dilated part of the 

 nasal duct, and lies in a depression formed by the 

 lacrymal and superior maxillary bones. (See Fig. 140, 

 page 399.) 



The nasal duct is contained in a canal formed by 

 the superior maxilla, lacrymal, and inferior turbinated 

 bones, and runs from the lacrymal sac to the inferior 

 meatus, beneath the inferior turbinate bone in the 

 nasal cavity. It is lined by a mucous membrane 

 continuous with the conjunctiva. 



The secretion from the lacrymal glands is a clear 

 fluid termed the tears, which is secreted by cells of 

 the gland due to sensor nerve fibers from the fifth 

 nerve, sending out impulses in response to reflex 

 stimuli, irritating the afferent nerves in the con- 

 junctiva, as foreign bodies in the eye, etc.; also by 



