THE CONJUNCTIVA 



859 



denser along the free edge of the lid, forming here the tarsus. The tarsus furnishes 

 insertion to a layer of unstriped muscle known as the tarsal muscle. The tarsal 

 glands (Glandulas tar sales) ^ are partly embedded in the deep face of the tarsus, and 

 are visible when the lid is everted if the conjunctiva is not too strongly pigmented. 

 They are arranged in a linear series, close together, and with their long axes per- 

 pendicular to the free edge of each lid. In the upper lid they number forty-five to 

 fifty; in the lower, thirty to thirty-five. Each consists of a tubular duct beset 

 with numerous alveoli, in which a fatty substance, the palpebral sebum, is secreted. 

 The palpebral conjunctiva lines the internal surface of the eyelids. 



^ The^ conjxinctiva is the mucous membrane which lines the lids as palpebral 

 conjunctiva (C. palpebrarum), and is reflected upon the anterior part of the eyeball 

 as bxilbar conjunctiva (C. bulbi); the line of reflection is termed the fornix con- 

 ■jimctivae. The palpebral part is closely adherent to the tarsus, but is loosely at- 

 tached further back. It is papillated and is covered with stratified cylindrical 

 epithelium in which many goblet-cells are present. In the fornix and its vicinity 

 there are tubular glands. Near the medial angle there are numerous lymph nodules. 

 The conjunctiva of the lateral part of the upper lid is pierced near the fornix by the 

 orifices of the excretory ducts of the lacrimal gland — twelve to sixteen in number. 



FiQ. 690. — Piece of Upper Eyelid; Inner Surface. 



1, Cilia; S, limbus palp, posterior; 3, tarsal glands. 



(After EUenberger, in Leisering's Atlas.) 



Fig. 691. — Cartilage of Third Eyelid of Horse; 



Convex Surface. 



1, Gland; S, fat surrounding deep part of cartilage. 



(After EUenberger, in Leisering's Atlas.) 



The bulbar conjunctiva is loosely attached to the anterior part of the sclera and is 

 pigmented in the vicinity of the corneo-scleral junction. On the cornea it is repre- 

 sented by a stratified epithelium. 



When the lids are in apposition, the conjunctiva (including the epithelium of the cornea) 

 encloses a capillary space between the lids and the eyeball, and constitutes what is known as the 

 conjunctival sac. 



The third eyelid (Palpebra tertia) is situated at the medial angle of the eye. 

 It consists of a semilunar fold of the conjunctiva, kno^vn as the membrana nictitans, 

 which covers and partly encloses a curved plate of hyaline cartilage. Its marginal 

 part is thin and usually more or less pigmented. The cartilage has an elongated 

 quadrilateral outline. The part of it which lies in the membrana is wide and thin. 

 The deep part is narrower and thicker and is embedded in fat at the inner side of the 

 eyeball. Numerous minute lymph nodules occur in the membrana nictitans, and 

 the deep part of the cartilage is surrounded by a gland which resembles the lacrimal 

 gland in structure (Glandula superficialis palpebrse tertise). 



Ordinarily the third eyelid extends very little over the medial end of the 

 cornea, but wJhen the eyeball is strongly retracted, the membrana is protruded over 

 it so as to measure about an inch (ca. 2-3 cm.) in its middle. This effect results 

 from the pressure of the eyeball and its muscles on the fat which surrounds the deep 

 part of the cartilage. 



Vessels and Nerves. — The arteries which chiefly supply the eyelids and con- 

 1 Also known as the Meibomian glands. 



