THE EYE. 407 



creases somewhat in size from above downwards, and at its 

 lower orifice is flattened from side to side. 



The lachrymal sac consists in two membranes; an exterior 

 fibrous one continuous with the periosteum of the contiguous 

 bones, and an interior mucous one, which is thick, villous.,of a 

 red colour, from the abundance of its blood vessels, and abound- 

 ing in mucous follicles. The interior is continuous above with 

 the lachrymal ducts, and below with the Schneiderian mem- 

 brane. On a line with the floor of the orbit, this internal mem- 

 brane is thrown into a circular duplicature, considered by some 

 anatomists as forming the proper boundary of the -lachrymal 

 sac : all below this is called by them Nasal Canal The dis- 

 tinction is rather arbitrary, and, in some degree, hurtful to clear- 

 ness of description: a much better plan is to call the part above 

 the valve the orbital portion of the sac, and the part below the 

 valve its nasal portion. Sometimes there is a second valve 

 about three lines below the first, and generally another at the 

 nasal orifice, formed by a duplication of the Schneiderian mem- 

 brane. 



The Tensor Tarsi is a small muscle on the orbital face of the 

 lachrymal sac, of which I gave a detailed account some years 

 ago.* It arises from the posterior superior part of the os unguis, 

 just in advance of the vertical suture between the os planum and 

 the os unguis. Having advanced three lines, it bifurcates; one 

 bifurcation is inserted along the upper lachrymal duct, and ter- 

 minates at its punctum, or near it; and the lower bifurcation has 

 the same relation to the lower lachrymal duct. The base of the 

 caruncula lachrymalis is placed in the angle of the bifurcation. 

 The superior and the inferior margins of the muscle touch the 

 corresponding fibres of the orbicularis palpebrarum, where the 

 latter is connected with the margin of the internal canthus of the 

 orbit, but may be readily distinguished by their horizontal 

 course. The nasal face of this muscle adheres very closely to 

 that portion of the sac which it covers, and also to the lachry- 

 mal ducts. The lachrymal sac rises about a line above its supe- 

 rior margin, and extends in the orbit four lines below its infe- 

 rior margin. The orbital face of the muscle is covered by a 



* Philadelphia Journal of Med. and Phys. Sciences, 1824. 



