THE PROTECTIVE ORGANS OF THE EYE. 473 



The blood-vessels of the eyelid lie directly in front of the tarsus, 

 and from this region supply adjacent parts ; they reach the poste- 

 rior portion of the lid either by penetrating the tarsus or by encir- 

 cling it (Waldeyer, 74). The lymph-vessels form a plexus in front 

 and one behind the tarsus. 



The " third eyelid," the plica semilunaris, contains, when well 

 developed, a small plate of hyaline cartilage. 



At the fornix the epithelium of the palpebral conjunctiva be- 

 comes continuous with the two- or three-layered squamous epithe- 

 lium of the conjunctiva bulbi. Beneath this epithelium is found a 

 loose fibro-elastic connective tissue, presenting subepithelial papillae, 

 and quite vascular. In it are found medullated nerve-fibers, some 

 of which terminate in free sensory nerve-endings in the conjunctival 

 epithelium ; others terminate, especially near the corneal margin, in 

 end-bulbs of Krause ; and still others may be traced to the cornea, 

 to terminate in a manner previously described. 



2. THE LACRIMAL APPARATUS. 



The lacrimal apparatus consists of the lacrimal glands, their ex- 

 cretory ducts, the lacrimal puncta and canaliculi, the lacrimal sac, 

 and the nasal duct. 



The lacrimal gland, which is a branched tubular gland, is sepa- 

 rated into two portions, of which the one lies laterally against the 

 orbit and the other close to the upper lateral portion of the superior 

 conjunctival fornix. The structure of the gland is, on the whole, 

 that of a serous gland (parotid), with the difference that the intralob- 

 ular ducts are not lined by a striated epithelium such as is found in 

 the salivary tubules, and that those cells which are wedged in between 

 the secretory elements and functionate as sustentacular cells (basket- 

 cells) are here much more highly developed. 



The excretory ducts of the orbital division generally pass by-the 

 conjunctival half of the gland, taking up a few ducts from the latter 

 as they go, and finally empty on the surface of the conjunctiva. 

 Aside from these, the lateral portion of the gland possesses also 

 independent ducts. All the excretory ducts are lined by columnar 

 epithelium and surrounded by a relatively thick connective-tissue 

 wall having inner longitudinal and outer circular fibers. From the 

 lateral portion of the conjunctival culdesac, into which the secre- 

 tion is brought by the excretory ducts of the lacrimal gland, the 

 secretion passes into the capillary space of the sac, and is then 

 evenly distributed by means of the sulci and papillae over the con- 

 junctival surface of the lid. In this manner the secretion reaches 

 the mesial angle of the lid, whence it passes through the lacrimal 

 puncta into the lacrimal canals. 



The nerve supply of the lacrimal glands is from the sym- 

 pathetic nervous system. The neuraxes of sympathetic neurones 

 accompany the gland ducts and form plexuses about the alveoli, 

 the terminal branches of which may be traced to the gland cells. 



