CHAP. ii.J SIGHT. 555 



conjunctiva, the nasal mucous membrane, tongue, optic nerve, &c. 

 or more directly by emotions. Venous congestion of the head 

 is also said to cause a flow. The efferent nerves belong either 

 to the cerebro-spinal system, (the lachrymal and orbital branches 

 of the fifth nerve,) or arise from the cervical sympathetic, the 

 afferent nerves varying according to the exciting cause. 



The act of blinking undoubtedly favours the passage of tears 

 through the lachrymal canals into the nasal sac, and hence when 

 the orbicularis is paralysed tears do not pass so readily as usual 

 into the nose ; but the exact mechanism by which this is effected 

 has been much disputed. According to some authors, the con- 

 traction of the orbicularis presses the fluid onwards out of the 

 canals, which, upon the relaxation of the orbicularis, dilate and 

 receive a fresh quantity. Others maintain that a special arrange- 

 ment of muscular fibres keeps the canals open even during the 

 closing of the lids, so that the pressure of the contraction of the 

 orbicularis is able to have full effect in driving the tears through 

 the canals. 



