THE LACHRYMAL APPARATUS. 847 



elastic in structure and somewhat dilated at their angle. The mucous membrane 

 is covered with scaly epithelium. 



The lachrymal sac is the upper dilated extremity of the nasal duct, and is 

 lodged in a deep groove formed by the lachrymal bone and nasal process of the 

 superior maxillary. It is oval in form, its upper extremity being closed in and 

 rounded, whilst below it is continued into the nasal duct. It is covered by a fibrous 

 expansion derived from the tendo oculi, and on its deep surface it is crossed by the 

 Tensor tarsi muscle (Horner's muscle, page 302), which is attached to the ridge on 

 the lachrymal bone. 



Structure. It consists of a fibrous elastic coat, lined internally by mucous 

 membrane, the latter being continuous, through the lachrymal canals'," with the 

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

 membrane of the nose. 



The nasal duct is a membranous canal, about three-quarters of an inch in 

 length, which extends from the lower part of the lachrymal sac to the inferior 

 meatus of the nose, where it terminates by a somewhat expanded orifice, provided 

 with an imperfect valve, the valve of ffasner, formed by the mucous membrane. It 

 is contained in an osseous canal formed by the superior maxillary, the lachrymal, and 

 the inferior turbinated bones, is narrower in the middle than at each extremity, 

 and takes a direction downward, backward, and a little outward. It is lined 

 by mucous membrane, which is continuous below with the pituitary lining of the 

 nose. This membrane in the lachrymal 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 elliptical in shape, 

 and differs in size in different individuals 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 eye- 

 ball 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 outward, so that the 

 outer angle is on a slightly higher level than the inner. This is especially noticeable in the Mon- 

 golian 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 circum- 

 stances. On the margins of the lids, about a quarter of an inch from the inner canthus, are two 

 small openings, the puncta lachrymalia, the commencement of the lachrymal canals. They are 

 best seen by everting the eyelids. In the natural condition they are in contact with the con- 

 junctiva 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 lachrymal canals. 

 The position of the lachrymal sac into which the canals open is indicated by a little tubercle 

 (page 119), which is plainly to be felt on the lower margin of the orbit. The lachrymal 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 position of the lachrymal sac may also be indicated 

 by the tendp oculi or internal tarsal ligament. If both lids be drawn outward, so as to tense 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 lachrymal sac, bisecting it, and thus forming a use- 

 ful 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 down- 

 ward through the duct into the inferior meatus of the nose. The direction of the duct is down- 

 ward, 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 the first molar tooth, 

 and should push his probe onward in such a way as if he desired to reach this structure. 



Beneath the internal angular process of the frontal bone the pulley of the Superior oblique 

 muscle of the eye can be plainly felt by pushing the finger backward between the upper and 

 inner angle of the eye and the roof of the orbit ; passing backward and outward trom this 

 pulley, the tendon can be felt for a short distance. 



Surgical Anatomy. The eyelids are composed of various tissues, and consequently are 

 liable to a variety of diseases. The skin which covers them is exceedingly thin and delicate, and 

 is supported on a quantity of loose and lax subcutaneous tissue which contains no tat. In conse- 

 quence of this it is very freely movable, and is liable to be drawn down by the contraction ..I 

 neighboring cicatrices, and thus produce an eversion of the lid known as ectropion. Inversion 



