THE SENSE OF SIGHT 727 



Accessory Structures. The eyeball is protected anteriorly by the 

 eyelids and their associated structures, the Meibomian glands, the lachrymal 

 glands, and tears. 



The eyelids consist of a central framework of connective tissue support- 

 ing 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 cutaneous edge of the lid is bordered with 

 short stiff hairs. At the inner extremity each eyelid presents a small opening, 

 the punctum lacrimale, the beginning of the lachrymal 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. Their ducts open on the free border of the lid. 

 These glands secrete an oleaginous material resembling sebaceous matter, 

 which accumulates along the margin of the lid and prevents the tears from 

 flowing down the cheek. 



The lachrymal 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 lachrymal secretion consists of 

 water and inorganic salts. It is distributed over the corneal surface during 

 the act of winking, thus keeping it moist and free from foreign particles. 

 It eventually passes into the lachrymal ducts and then into the nose. The 

 lachrymal 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. 



