= 
698 THE ORGANS OF SENSE. 
and nasal process of the superior maxilla, and ends above in a rounded, blind 
extremity or fundus, while it narrows below into the nasal duct. Here a fold of 
mucous membrane, named the valve of Beraud or of Krause, together with a later- 
ally-directed pouch, the sinus of Arlt, are sometimes present. Near its superior 
extremity it 1s crossed in front by the internal tarsal ligament, from the upper 
and lower edges of which the orbicularis palpebrarum takes origin, while behind 
it is the tensor tarsi muscle, or muscle of Horner. 
The nasal duct (ductus naso-lacrimalis) averages about 18 mm. in length and 
has a diameter of from 5-4mm. Rather narrower near its middle than at its upper 
and lower extremities, it is directed downwards and sheghtly backwards, and opens 
into the inferior meatus of the nose at the junction of its anterior with its pos- 
terior three-fourths, 7.¢. a distance of 50-55 min. from the posterior boundary of the 
nostril. Its lower orifice is somewhat variable in form and position, and is occa- 
sionally dupheated. It is frequently guarded by a fold of mucous membrane, termed 
the valve or plica lacrimalis of Hasner. Through this orifice the mucous lining of 
the duct is continuous with that of the nose. 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 lachrymal canals and sac are derived from the infratrochlear branch 
of the nasal; their arteries from the inferior palpebral and nasal. The veins of the 
nasal duct are large and numerous, forming a sort of erectile tissue similar to that 
in the nose. 
DEVELOPMENT OF THE EYE. 
The retina and optic nerve aredeveloped from a hollow outgrowth of the fore-brain, termed 
] : ; 
the optic vesicle (see pp. 442 and 553). 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 
Lens rudiment 
Optic cup 
Optic stalk | 
Cavity of 
fore-brain — 
Eetoderm 
forming 
lens 
rudiment 
f Sy x : 
é os q Outer layer of optie cup 
fee ‘3 1 Inner layer of optic cup 
anise] SK sone 
x ‘ew Lens 
Optic Ree 7 
vesicle “— 
becoming 
eupped 
| Optie stalk 
Chorioidal fissure 
Lens 
Fic. 507.--SECTIONS THROUGH PORTIONS OF THE HEADS or Fara Rappirs, to illustrate the connexion 
of the optic cup with the fore-brain, and the invagination of the ectoderm to form the lens. 
island of cells, which is developed into the lens and is named the lens vesicle. This lens. 
rudiment indents the outer and lower part of the optic vesicle, which now assumes the form 
of 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 its pigmented epithelium. 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 pars ciliaris and pars iridica retinee. The in- 
dentation of the optic cup extends as a groove for some distance along the postero-inferior 
