724 SPECIAL SENSES. 



sels, nerves and a part of the lachrymal apparatus ; and it contains, also, a cer- 

 tain quantity of adipose tissue, which latter never disappears, even in extreme 

 marasmus. The bony walls of this cavity protect the globe and lodge the 

 parts above enumerated. The internal, or nasal wall of the orbit projects 

 considerably beyond the external wall, so that the extent of vision is far 

 greater in the outward than in the inward direction. As the globe is more 

 exposed to accidental injury from an outward direction, the external wall of 

 the orbit is strong, while the bones which form its internal wall are compara- 

 tively fragile. The upper border of the orbit (the superciliary ridge) is pro- 

 vided with short, stiff hairs (the eyebrows) which serve to shade the eye from 

 excessive light and to protect the eyelids from perspiration from the fore- 

 head. 



The eyelids are covered by a very thin integument and are lined by the 

 conjunctival mucous membrane. The subcutaneous connective tissue is thin 

 and loose and is entirely free from fat. The skin presents a large number of 

 short papillae and small, sudoriparous glands. At the borders of the lids, are 

 short, stiff, curved hairs, arranged in two or more rows, called the eyelashes 

 or cilia. Those of the upper lid are in greater number and longer than the 

 lower cilia. The curve of the lashes is from the eyeball. They serve to pro- 

 tect the globe from dust, and to a certain extent, to shade the eye. 



The tarsal cartilages are small, elongated, semilunar plates, extending 

 from the edges of the lids toward the margin of the orbit, between the skin 

 and the mucous membrane. Their length is about an inch (25'4 mm.). The 

 central portion of the upper cartilage is about one-third of an inch (8'5 mm.) 

 broad, and the corresponding part of the lower cartilage measures about one- 

 sixth of an inch (4-2 mm.). At the inner canthus, or angle of the eye, is a 

 small, delicate ligament, or tendon, the tendo palpebrarum, which is attached 

 to the lachrymal groove internally, passes outward, and divides into two 

 lamellae, which are attached to the two tarsal cartilages. At the outer can- 

 thus the cartilages are attached to the malar bone, by the external tarsal liga- 

 ment. The tarsal cartilages receive additional support from the palpebral 

 ligament, a fibrous membrane attached to the margin of the orbit and the 

 convex border of the cartilages and lying beneath the orbicularis muscle. 

 This membrane is strongest near the outer angle of the eye. 



On the posterior surface of the tarsal cartilages, partly embedded in them 

 and lying just beneath the conjunctiva, are the Meibomian glands. The 

 structure and uses of these glands have already been described in connec- 

 tion with the physiology of secretion. They produce an oily fluid, which 

 smears the edges of the eyelids and prevents the overflow of tears. 



Muscles which open and close the Eyelids. The corrugator supercilii 

 draws the skin of the forehead downward and inward ; the orbicularis palpe- 

 brarum closes the lids ; and the levator palpebrae superioris raises the upper 

 lid. The tensor tarsi, called the muscle of Homer, is a very thin, delicate 

 muscle, which is regarded by some anatomists as a deep portion of the orbic- 

 ularis. Considering this as a distinct muscle, it consists of two delicate 

 slips, which pass from either eyelid, behind the lachrymal sac, uniting here 



