1080 SPECIAL SENSE ORGANS 



courses more or less horizontally, converging slightly toward its fellow, and not 

 infrequently joining it before opening into the sac. The calibre varies consider- 

 ably in this course, being narrowest a short distance from the punctum, and widest 

 at the bend, from which point it again narrows very gradually as it nears the sac. 



The wall of the ductus consists mainly of elastic and white fibrous tissue, lined internally 

 by epithelium, and covered externally by striated muscle (part of the orbicularis). The muscle- 

 fibres run parallel to the ductus in the horizontal part of its course; but they are placed, some 

 in front and some behind, around the vertical part, acting here as a kind of sphincter. Just 

 before their termination, the ducts pierce the periosteal thickening that constitutes the posterior 

 limb of the medial palpebral ligament. 



The lacrimal sac [saccus lacrimalis] lies in a depression in the bone at the medial 

 angle of the orbit (the lacrimal fossa). It is vertically elongated, and narrows at 

 its upper and lower ends; the upper extremity or fundus is closed, while the lower 

 is continuous with the naso-lacrimal duct. Laterally, the sac is somewhat com- 

 pressed, so that its antero-posterior is greater than its transverse diameter. The 

 ducts, either separately or by a short common tube, open into a bulging on the 

 lateral surface of the sac near the fundus. 



As has previously been mentioned, the sac is surrounded by periosteum, but between this 

 and the mucous membrane forming the true sac-wall there is a loose connective tissue, so that 

 the cavity is capable of considerable distention. The relations of the medial palpebral ligament 

 have already been described; it is to be noted that the fundus of the sac extends above this 

 ligament. 



The naso-lacrimal duct [ductus naso-lacrimalis] reaches from the lower end of 

 the sac to the top of the inferior meatus of the nose, opening into the latter just 

 beneath the adherent border of the inferior nasal concha. Traced from above, 

 its main direction is downward, but it has also a slight inclination backward and 

 laterally. It lies in a bony canal, whose periosteum forms its outer covering. 

 Between this and the mucous membrane of the duct there is a little intermediate 

 tissue, in which run veins of considerable size connected with the plexus of the 

 inferior concha. The duct does not usually open directly into the nasal cavity 

 at the lower end of the bony canal, but pierces the nasal mucous membrane very 

 obliquely, so that a, flap [plica lacrimalis (Hasneri)] of mucous membrane covers 

 the lower border of the opening in the bone, upon which flap the tears first trickle 

 after escaping from the duct proper. 



The sac and naso-lacrimal duct together constitute the lacrimal canal, lined throughout by 

 a continuous mucous membrane. This membrane presents folds in some situations, especially 

 near the opening of the canaliculi, at the junction of the sac and duct, and at the lower end of 

 the duct. That at the top of the duct is the most important, as it sometimes interferes with the 

 proper flow of tears out of the sac. The total length of the lacrimal canal is roughly twenty-four 

 millimetres, half of this being sac, and half naso-lacrimal duct. If, however, we reckon as duct 

 the oblique passage through the nasal mucous membrane, this measurement may occasionally 

 be increased by eight or ten millimetres. Tlie lacrimal sac, when distended, measures about 

 six millimetres from before backward, by four millimetres transversely. The naso-lacrimal duct 

 is practically circular, and has a diameter of about three millimetres, rather less at its junction 

 with the sac, where we find the narrowest part of the whole lacrimal canal. 



Development of the Eye 



The eye is developed from the three sources involving two fundamental embryonic layers — 

 the retina from a portion of the ectodermal wall of the forebrain on each side; the lens from the 

 ectod(!rmal surface epithelium; and the sclera, cornea (except epithelium) and chorioidal 

 coat from the mesoderm which surrounds the former structures. 



The process of development is, briefly, as follows: — The site of the eye is marked by a slight 

 depression on the surface of the forebrain on either side. There later an outgrowth occurs 

 from the ventro-lateral aspect on each side of the forebrain, in the form of a hollow vesicle, 

 whose cavity is continuous with that of the forebrain. This outgrowth is termed the primary 

 optic vesicle [vosicula oplithalniica]. The lateral surface of the vesicle comes into contact with 

 the surface epithelium of the head and this epithelium becomes thickened at the area of contact. 

 The superficial portion of the vesicle expands, while its connection with the brain remains 

 slender; becoming depressed on the surface, it forms a cup-shaped hollow, the secondary optic 

 vesicle or optic cup [calicuhis ophthalmicus] whose wall is formed of two layers, an outer investing 

 layer and an inner inverted one. 



The chorioidfd fissure is present almost from the first stapes, as a cleft on the ventral aspect 

 of both the distal portion of the vesicle, or cup, and of the stalk; and it is formed by an infolding 

 of the surface into the cavity of the vesicle along a narrow linear area. 



In this cleft aro found vessels which pa.ss to the hollow of the optic cup. The margins of 

 the cleft meet and fuse, and enclose the vessels in the interior^hence the enclosure of the 

 a. centralis retin.-p witliin the of)tic nerve, and of the hyaloid artery in the interior of the vitreous. 

 Should the margins of the cleft remain separate, the condition of coloboma results. 



