372 



NORMAL HISTOLOGY. 



mal canals are surrounded by a layer of striped muscle derived 

 from that part of the orbicularis known as Horner's muscle ; this 

 tissue is arranged as small bundles, which possess a general longitu- 

 dinal course parallel with the axis of the greater part of the lachry- 

 mal canals. The vertical papillary division of the tube, however, 

 lies at right angles to the muscle-bundles, which, consequently, seem 

 to enclose this part of the canal within circular or sphincter fibres ; 

 some of these occupy the edge of the lid and surround the puncta 

 with muscular loops. 



The mucous membrane of the lachrymal sac and of the naso- 

 lachrymal duct is connected with the periosteum of the neighbor- 

 ing bony surfaces by loose areolar tissue, within which is lodged a 

 rich venous plexus. 



The mucous membrane of the lachrymal sac and of the duct 

 partakes largely of the nature of lymphoid tissue, consisting of a 

 connective-tissue reticulum infiltrated with lymphoid cells. From 

 the tear-sac to the nasal termination of the duct the lining epithe- 

 lium is stratified columnar in character, with the occasional pres- 

 ence of cilia within the lower part of the tube. 



The eyeball is separated from the surrounding structures within 

 the orbit by the intervention of a fibro-elastic membrane or fascia, 

 the capsule of Tenon, covered by a continuous layer of endothe- 

 lial plates ; the enclosed episcleral space, or space of Tenon, 

 communicates with the perichoroidal space on the one hand and 

 with the supra- vaginal cleft on the other. In effect, the capsule of 

 Tenon corresponds to a synovial sac, whose lubricated surfaces of 

 contact facilitate the movements of the eyeball. 



DEVELOPMENT OF THE EYE. 



The earliest indication of the visual organ is the optic vesicle, a 

 large diverticulum extending on either side from the primary anterior 

 brain-vesicle, and later becoming connected by a constricted stalk 

 with the interbrain, or thalamencephalon. 



In the early stage the optic vesicle lies in contact with the ectoderm 

 reflected over the prominently protruding optic diverticulum, the sur- 

 rounding mesoderm at first showing no differentiation. Shortly after 

 the optic vesicle has reached the surface ectoderm the latter exhibits 

 proliferation and thickening opposite the external pole of the vesicle. 

 This ectodermic area, the earliest trace of the future crystalline 

 lens, soon becomes depressed, the invagination progressing until 

 the pit- and the cup-stage give place to the closed vesicle, which 

 finally separates from the ectoderm and lies beneath the surface as 

 the lens-sac. 



Simultaneously with the progress of these changes in the ectoderm, 



