636 TEXT-BOOK OF PHYSIOLOGY. 



The eyelids consist of a central framework of connective tissue 

 supporting muscle tissue (the orbicularis palpebrarum muscle) and 

 glands, and covered externally by skin and internally by a modified 

 skin, the conjunctiva. The free border of each lid is strengthened 

 by a semilunar plate of dense fibrous tissue, the tarsus. The cuta- 

 neous edge of the lid is bordered with short stiff hairs. At the inner 

 extremity each eyelid presents a small opening, the punctum lacri- 

 malis, the beginning of the lacrimal duct. The two ducts after 

 uniting open into the nasal duct. 



The Meibomian glands are modified sebaceous glands imbedded 

 in the posterior portion of the lids (Fig. 308). Their ducts open on 

 the free border of the lid. These glands secrete an oleaginous ma- 

 terial resembling sebaceous matter which accumulates along the 

 margin of the lid and prevents the tears from flowing down the 

 cheek. 



The lacrimal gland is situated at the upper and outer part of the 

 orbit cavity. It consists of a series of compound tubules lined by 

 epithelium. The secretion (the tears) is conducted from the gland 

 to the outer part of the conjunctiva by seven or eight ducts. The 

 lacrimal secretion consists of water and inorganic salts. It is dis- 

 tributed over the corneal surface during the act of winking, thus 

 keeping it moist and free from foreign particles. It eventually passes 

 into the lacrimal ducts and then into the nose. The lacrimal glands 

 receive secretory fibers by way of the fifth nerve and the cervical 

 sympathetic. The secretion can be excited reflexly from stimulation 

 of sensor nerves as well as by emotional states. 



