THE APPENDAGES OF THE EYE 



1119 



surface it is crossed by the Tensor tarsi muscle (Horner's muscle, p. 367), which 

 is attached to the ridge on the lacrimal bone. 



Structure. The lacrimal sac consists of a fibrous elastic coat, lined internally by mucous 

 membrane, the latter being continuous, through the ampulla and lacrimal canals, with the 

 mucous lining of the conjunctiva, and, through the nasal duct, with the mucous membrane of 

 the nose. 



The Nasal Duct (ductus nasolacrimalis} (Fig. 828) is a membranous canal, 

 about 2 cm. (three-quarters of an inch) in length, which extends from the lower 

 part of the lacrimal sac to the inferior meatus of the nose, where it terminates by 

 a somewhat expanded orifice, provided with an imperfect valve, the valve of 

 Hasner (plica lacrimalis '[Hasneri]), 

 formed by a fold of mucous mem- 

 brane. It is contained in an osseous 

 canal formed by the maxilla, the lac- 

 rimal. and the turbinated bones, is 

 narrower in the middle than at each 

 extremity, a nd ta kesl^iirecuonuown- 

 ward, backward, and a little out- 

 ward. It is lined by mucous mem- 

 brane, which is continuous below with 

 that of the nose. The membrane in 

 the lacrimal sac and nasal duct is 

 covered with columnar epithelium, as 

 in the nose; this epithelium is in places 

 ciliated. 



Surface Form. The palpebral fissure, or 

 opening between the eyelids, is elliptic in 

 shape, and differs in size in different individ- 

 uals and in different races of mankind. In 



the Mongolian races, for instance, the opening is very small, merely a narrow fissure, and this 

 makes the eyeball appear small in these races, whereas the size of the eye is relatively very 

 constant. The normal direction of the fissure is slightly oblique, in a direction upward and out- 

 ward, so that the outer angle is on a slightly higher level than the inner. This is especially 

 noticeable in the Mongolian races, in whom, owing to the upward projection of the malar bone 

 and the shortness of the external angular process of the frontal bone, the tarsal plate of the 

 upper lid is raised at its outer part and gives an oblique direction to the palpebral fissure. 

 When the eyes are directed forward, as in ordinary vision, the upper part of the cornea is 

 covered by the upper lid, and the lower margin of the cornea corresponds to the level of the lower 

 lid, so that about the lower three-fourths of the cornea is exposed under ordinary circumstances. 



On the margin of the lids, about 1 cm. from the inner canthus, are two small openings, the 

 puiicta lacrimalia, the commencement of the lacrimal canals. They are best seen by everting 

 the eyelids. In the natural condition they are in contact with the conjunctiva of the eyeball, 

 and are maintained in this position by the Tensor tarsi muscle, so that the tears running over the 

 surface of the globe easily find their way into- the lacrimal canals. The position of the lacrimal 

 sac into which the canals open is indicated by a little tubercle, which is plainly to be felt on the 

 lower margin of the orbit. The lacrimal sac lies immediately above and to the inner side of this 

 tubercle, and a knife passed through the skin in this situation would open the cavity. The posi- 

 tion of the lacrimal sac may also be indicated by the tendo oculi or internal tarsal ligament. If 

 both lids be drawn outward, so as to tighten the skin at the inner angle, a prominent cord will be 

 seen beneath the tightened skin. This is the tendo oculi, which lies immediately over the lacrimal 

 sac, bisecting it, and thus forming a useful guide to its situation. A knife entered immediately 

 beneath the tense cord would open the lower part of the sac. A probe introduced through this 

 opening can be readily passed downward through the duct into the inferior meatus of the nose. 

 The direction of the duct is downward, outward, and backward, and this course should be borne 

 in mind in passing the probe, otherwise the point may be driven through the thin bony walls of 

 the canal. A convenient plan is to direct the probe in such a manner that if it were pushed 

 onward it would strike the first molar tooth of the lower jaw on the same side of the body. In 

 other words, the surgeon standing in front of his patient should carry in his mind the position of 



FIG. 828. The lacrimal apparatus. Right side. 



