408 NERVOUS SYSTEM. 



lamina of cellular membrane, and between this lamina and the 

 ball of the eye are placed the valvula semilunaris, and a consi- 

 derable quantity of adipose matter. 



As the bifurcated extremities of the muscle follow the course 

 of the ducts, they are covered by the tunica conjunctiva. When 

 this muscle is examined from behind, the eyelids being in situ, 

 it becomes obvious that it is concave on its orbital surface, and, 

 consequently, convex on the nasal; that the muscle is an oblong 

 body, half an inch in length, and about three lines wide, bifur- 

 cated at one end: and that it arises much deeper from the orbit 

 than any acknowledged origin of the orbicularis. The superior 

 fork, however, has a few of its fibres blended with the orbicu- 

 laris. 



In regard to the use of this muscle: its attachment to the pos- 

 terior face of the sac is such, that it draws the orbital parts of 

 the sac away from the nasal, and dilates the sac, from the nasal 

 face of the latter being fixed to the bones. As this muscle is 

 cylindrically concave on its orbital side, it is evident that when 

 it contracts, the fibres become straight, or nearly so, like the 

 fibres of the diaphragm, and the cavity of the sac is enlarged 

 after the same manner as the cavity of the thorax. A tendency 

 to a vacuum being thus produced by it, the valves or folds of 

 the internal membrane of the sac permit the vacuum to be filled 

 more readily through the puncta than from the nose; and the 

 puncta being continually bathed in the tears of the lacus lachry- 

 malis, both in the waking and in the sleeping state, the tears are 

 constantly propelled through them by atmospheric pressure. 

 The evacuation of the sac is no doubt accomplished by its own 

 elasticity, and by the contraction of the orbicularis; probably 

 in a chief degree by the latter, because in persons who have 

 epiphora, or a tendency to obstruction in the nasal duct, the ac- 

 cumulation of tears and matter principally takes place at night, 

 when the action of the orbicularis is suspended by sleep. For 

 these reasons, we should argue that this little muscle is active at 

 all times, both night and day. To Dr. Physick I am indebted 

 for suggesting another use for it; to wit, that of keeping the lids 

 in contact with the ball of the eye. 



Some persons possess unusual voluntary powerover this muscle, 

 of which I have seen several examples. In each instance the in- 



