406 NERVOUS SYSTEM. 



ated immediately at the outer margin of the caruncle. It has 

 the form of a triangle, the point of which runs into the caruncle, 

 and the base, which is somewhat crescentic, is directed towards 

 the eye. It is a duplicature of the conjunctiva, becomes very 

 manifest from behind, and, in some persons, has its base fur- 

 nished with a very small strip of cartilage.* I have seen seve- 

 ral cases of the kind very well marked; they occur more 

 frequently in the African, according to my personal experience. 

 Between the base and the caruncle it is formed into a number of 

 loose wrinkles, which disappear when the eye is very much ab- 

 ducted. 



This body, in the human subject, is evidently intended to per- 

 mit to the eye a great freedom of abduction. Its analogy, how- 

 ever, with the third eyelid of animals is very striking, and the 

 difference is rather in the full development of the latter, than in 

 the organization. It is very properly remarked by J. F. Meckel, 

 that in descending the scale of animals, the third eyelid is al- 

 ways in an inverse ratio to the other two, till it ends by being 

 a complete substitute for them. 



Surrounding the Lachrymal caruncle may be observed a de- 

 pression on the adjoining part of the plica semilunaris. Into 

 this depression the puncta lachrymalia are directed, and there 

 play up and down: the tears accumulate in it; from which cause 

 the depression is called Lacus Lachrymalis. 



The Lachrymal Sac (Saccus Lachrymalis) is placed at the in- 

 ternal canthus of the orbit; in the depression of the os unguis, 

 and of the nasal process of the upper maxillary bone. It is an 

 oblong cylindroid cavity, and extends from the transverse facial 

 suture to the anterior extremity of the inferior meatus of the 

 nose; being concealed there by the anterior part of the inferior 

 turbinated bone. It is crossed at its front part by the tendon of 

 the orbicular ; s, which, with a few fibres of this muscle, adheres 

 to it. 



The course of the lachrymal sac is, at first, slightly forwards 

 in the descent to the nose; but, when it reaches the lower part 

 of the orbit, it is afterwards slightly backwards: so that it may 

 be considered as forming an obtuse angle forwards. It also de- 



* Meckel. 



