736 THE SENSE ORGANS AND SKIN OF THE HORSE 



minute, slit-like opening, the punctum lacrimalis, which is the entrance to the 

 lacrimal duct or canaliculus. 



Structure of the Eyelids. — The skin of the eyelids is thin and freely movable, 

 except near the free edge, where it is more firmly attached. The underlying 

 subcutaneous tissue is destitute of fat. The muscular layer consists chiefly of the 

 elliptical bundles of the orbicularis oculi, with which are associated fibers of the 

 corrugator supercilii in the upper lid and the malaris in the lower lid. At the inner 

 side there is a fibrous l)and, the palpebral ligament, which is attached to the lacrimal 

 tubercle and furnishes origin to some fibers of the orbicularis. At the inner com- 

 missure a bundle detached from the orbicularis passes inward behind the lacrimal 

 sac, and is known as the pars lacrimalis or Horner's muscle. At the outer side an 

 indistinct palpebral raphe occurs where fibers of the orbicularis decussate. The 

 fibrous layer is thicker and 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 (of Miiller). The tarsal glands (Glandulse tarsales) are partly 

 embedded in the deep face of the tarsus, and are visil)le 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 perpendicular to the free edge of 

 each lid. In the upper lid they number forty-five to fifty; in the lower, thirt}' to 

 thirty-five. Each consists of a tubular duct beset with numerous alveoli, in w^iich a 



Fig. 553.— Piece of Upper Eyelid, Inner Surface. Fig. 554. — C.\rtilage of Third Eyelid of Horse, 

 1, Cilia; 2, limbus palp, posterior; 3, tareal glands. Convex Surface. 



(After Elleiiberger, in Leisering's Atlas.) 1, Gland; 2, fat surrounding deep part of cartilage. 



(.After EUenberger, in Leisering's Atlas.) 



fatty substance, the palpebral sebum, is secreted. The palpebral conjunctiva lines 

 the posterior surface of the eyelids. 



The conjunctiva is the mucous membrane which lines the lids as palpebral 

 conjunctiva (C. ]ialpe]irarum) and is reflected upon the anterior part of the eyeball 

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

 junctivae. 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 (of Henle). Near the inner angle there are numerous 

 lymph nodules. The conjunctiva of the outer part of the upper lid is pierced near 

 the fornix by the orifices of the excretory ducts of the lacrimal gland. 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 represented by a 

 stratified epithelium. 



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

 forms a complete conjunctival sac, as the capillary space l)et\vcen the lids and the eyeball 

 is termed. 



The third eyelid (Pal])ebra tertia) is situated at the inner angle of the eye 

 (Figs. 551, 556). It consists of a semilunar fold of the conjunctiva, known as the 

 membrana nictitans, which covers and partly incloses a curved plate of hj^aline 

 cartilage. Its marginal part is thin and usually more or less pigmented. The 



