1048 ORGANS OF SPECIAL SENSE 



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 inner 

 palpebral ligament. 



The lachrymal sac lies in a depression in the bone at the inner angle of the 

 orbit (the lachrymal fossa). It is vertically elongated, and narrows at its upper 

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

 uous directly with the nasal duct. Laterally, the sac is somewhat compressed, 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 outer sur- 

 face 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 distension. The relations of the inner palpebral ligament have 

 already been described; if is to be n'oted that the fundus of the sac extends above 

 this ligament. 



The naso-lachrymal duct 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 down- 

 wards, but it has also a slight inclination backwards and outwards. 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 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-lachrymal duct together constitute the lachrymal canal, 

 lined throughout by a continuous mucous membrane. This membrane presents 

 folds in some situations, especially near the opening of the canaliculi, at the junc- 

 tion 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 lachrymal canal is roughly twenty-four 

 millimetres, half of this being sac, and half naso-lachrymal 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. The 

 lachrymal sac, when distended, measures about six millimetres from before back- 

 wards, by four millimetres transversely. The naso-lachrymal 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 lachrymal canal. 



