588 THE EYE. 



The lachrymal sac and nasal duct constitute together the passage 

 by which the tears are conveyed from the lachrymal canals to the 

 cavity of the nose. The lachrymal sac (fig. 398, 2), the slightly dilated 

 upper portion of the passage, is situated at the side of the nose, near 

 the inner canthus of the eye, and lies embedded in a deep groove 

 in the ungual and upper maxillary bones. Its upper end is closed 

 and rounded, and the lower end gradually narrows into the nasal 

 duct. On the outer side, and a little in front, it receives the 

 lachrymal canals ; and here it is placed behind the tendo palpebrarum, 

 and some cf the inner fibres of the orbicular muscle of the lids ; 

 while on its orbital surface is the tensor tai'si muscle. The sac is 

 composed of fibrous and elastic tissues, adhering closely to the 

 bones above mentioned, and strengthened by fibrous processes sent 

 from the tendo palpebrarum, which crosses a little above its middle. 

 The inner sm'face is lined by a reddish mucous membrane, which is 

 continuous through the canaliculi with the conjunctiva, and through 

 the nasal duct with the raucous membrane of the nose. 



The nasal duct (ductus ad nasum), about six or seven lines in length, 

 grooving the upper maxillary bone, descends to the fore part of the 

 lower meatus of the nose, the osseous canal being completed by the 

 ungual and lower turbinated bones. A tube of fibrous membrane, con- 

 tinuous with the lachrymal sac, adheres to the parietes of this canal, and 

 is lined by mucous membrane, which, at the opening into the nose, is 

 often arranged 'so as to form an imperfect valve. The nasal duct is 

 rather narrower in the middle than at either end ; its direction is not 

 quite vertical, but inclined slightly outwards and backwards. 



The mucous membrane in the canaliculi possesses a stratified, scaly 

 epithelium, but in the nasal sac and duct a cihated epithelium as in the 

 nose. 



Various valves have been described in connection with the lachrjanal sac and 

 canals. One. the valve of Hasner, is formed by the mucous membrane of the 

 nose overhanging the inferior orifice of the nasal diict, and has had imputed to 

 it the function of preventing entrance of foreign matters in violent expiratory- 

 movements ; but the disposition of the mucous membrane at this orifice appears 

 to be subject to considerable variation. Another fold, the valve of Husehke, 

 placed at the opening of the canaliculi into the lachrjanal sac, is supposed by some 

 to prevent the return of the tears from the sac into those tubes, but, by others, 

 it is declared to be mcoustant, and insufiicient, even when present, to close the 

 orifice. A third fold, the valve of Foltz, is described as forming a projection 

 inwards on one side of the vertical part of each canaliculus, near the ptmctum 

 lachrymale. and as being sufficient to close the tube when it is flattened by the 

 pressure of the fibres of the orbicularis and tensor tarsi muscles as in winking. 

 The experiments of Foltz on rabbits go to prove that the punctum lachrj-male 

 having been tiurned backwards towards the eye in winking, and the canaliculus 

 being compressed bj^ the mtiscles, as soon as the pressure is removed the canali- 

 culus resumes its open form, and so sucks in tears which by the next compression 

 in winking are forced onwards into the lachrymal sac : and also, that when the 

 muscles are jDaralysed, the canaliculi cease to carry away the tears. See review 

 of Foltz's paper in Dublin Quarterly Jomnal, Feby. l«6o ; also, Hyrtl, Topogr. 

 Anatomie. 



THE GLOBE OF THE EYE. 



The globe or ball of the eye is a composite structure of spheroidal 

 form, placed in the fore part of the orbital cavity, and receiving the 



