

DEVELOPMENT OF THE EYE. 825 



is supplied by the sympathetic and lacrinial nerves and by the lacrimal artery, 

 while its veins are drained into the ophthalmic vein. 



Ductus Lacrimales. The lacrimal ducts, two in number, commence in minute 

 orifices, termed the puncta lacrimalia, at the apices of the papillae lacrimales 

 (p. 821), and are directed medial wards, along the medial parts of the margins of the 

 eyelids, above and below the lacus lacrimalis. The superior duct at first ascends 

 for a short distance and then inclines downwards ; the inferior duct descends for 

 a short distance and then runs horizontally; at the angle where it changes its 

 direction each duct is dilated into an ampulla. The two ducts open close together 

 into the lateral and front part of the lacrimal sac, a little below its middle ; some- 

 times they open separately into a pouch-like dilatation of the sac, termed the sinus 

 of Maier. The ducts are lined with stratified epithelium placed on a tunica propria, 

 outside which is a layer of striped muscular fibres derived from the lacrimal part 

 of the orbicularis oculi. These muscular fibres are arranged somewhat spirally 

 around the ducts, but at the base of each papilla lacrimalis they are circular 

 in direction and form a species of sphincter. On contraction they serve to 

 empty the contents of the lacrimal ducts into the lacrimal sac. ' 



The saccus lacrimalis and ductus nasolacrimalis together form the passage by 

 which the tears are conveyed from the lacrimal ducts to the nasal cavity. 



The lacrimal sac is the upper expanded part of the passage, and measures from 

 12 to 15 mm. in length, about 7 mm. antero-posteriorly, and from 4 to 5 mm. trans- 

 versely. It lies in the groove formed by the lacrimal bone and frontal process 

 of the maxilla, and ends above in a rounded, blind extremity or fundus, while 

 it narrows below into the naso-lacrimal duct. At the junction with the duct a 

 fold of mucous membrane, named the valve of Beraud, together with a laterally 

 directed pouch, the sinus of Arlt, are sometimes present. Near its superior ex- 

 tremity it is crossed, in front, by the medial palpebral ligament (O.T. tendo oculi), 

 from the upper and lower edges of which the orbicularis oculi takes origin ; the 

 lacrimal part of the orbicularis oculi muscle is behind it. 



The naso-lacrimal duct averages about 18 mm. in length, and has a diameter of 

 from 3 to 4 mm. Eather narrower in the middle than at its extremities, it is directed 

 downwards and slightly backwards, and opens into the inferior meatus of the nose 

 at the junction of its anterior fourth with its posterior three-fourths, i.e. a distance 

 of 30 to 35 mm. from the posterior boundary of the nostril. Its lower orifice is 

 somewhat variable in form and position, and is occasionally duplicated. It is 

 frequently guarded by a fold of mucous membrane, termed the plica lacrimalis 

 (Hasneri). Through this orifice the mucous lining of the duct is continuous with 

 that of the nasal cavity. The mucous membrane of the duct is thrown into 

 inconstant folds, several of which have been described as valves. Its epithelium 

 is columnar and in part ciliated ; opening into the lower part of the duct are 

 numerous glands, similar to those in the nasal mucous membrane. 



The nerves of the lacrimal ducts and sac are derived from the infra-trochlear branch of the 

 naso-ciliary ; their arteries from the inferior palpebral and nasal. The veins of the naso-lacrimal 

 duct are large and numerous, forming a sort of erectile tissue similar to that in the nasal cavity. 



DEVELOPMENT OF THE EYE. 



The retina and optic nerve are developed from a hollow outgrowth of the fore-brain, termed 

 the optic vesicle (see pp. 54 and 33). This extends towards the side of the head, and its 

 connexion with the brain is gradually elongated to form the optic stalk. The ectoderm 

 overlying the optic vesicle becomes thickened, invaginated, and finally cut off as a hollow 

 island of cells, which is named the lens vesicle. This vesicle indents the outer and lower 

 part of the optic vesicle, converting it into a cup (optic cup), lined by two layers of cells 

 continuous with each other at the margin of the cup. The inner of these strata, thicker 

 than the outer, is named the retinal layer, and becomes differentiated into the nervous 

 and supporting elements of the retina ; while the outer, named the pigmentary layer, 

 forms the stratum pigmenti. The edge of the optic cup extends in front of the equator 

 of the lens, and bounds the future aperture of the pupil. In front of the lens, and also 

 opposite its equator, the retinal layer is thin, and represented only by a stratum of 

 columnar cells which becomes closely applied to the pigmentary layer, the two forming the 



