SENSITIVE SYSTEM. 375 



Ductus ad Nasum. 



The ductus ad nasum is a long membranous canal, be- 

 ginning from the contracted bottom of the lachrymal sac, run- 

 ning within the groove through the lachrymal bone ; then along 

 a canal in the superior maxillary bone, between it and the ante- 

 rior turbinated bone, to terminate at the inner and inferior part of 

 the nostril, upon the cuticular surface there, by an oblique oval 

 orifice, which, from being constantly open, conspicuously strikes 

 our view whenever we dilate the nostril. The precaution of its 

 termination upon the cuticular surface was evidently taken to 

 prevent the tears from trickling upon and irritating the mem- 

 brane of the nose. In the first part of its course the duct di- 

 minishes a little in its caliber ; from about its middle, however, it 

 begins to enlarge again, and soon after acquires its former dia- 

 meter. After having emerged from its bony passages, the duct 

 makes several flexures, which, though inconsiderable, are suffi- 

 cient to render the introduction of a probe through it at this 

 part an affair of some difficulty. The membrane composing 

 the duct appears to be a continuation of tliat which lines the sac: 

 in its passage through the bones it is strengthened by a fibrous 

 sheath. Its internal surface is shielded from the salineness of 

 the tears by a mucous secretion. Its office is that of conveying 

 away the tears collected within the sac, and of discharging them 

 at the external nostrils. 



Course of the Tears. — The tears, as they issue from the ducts 

 of the lachrymal gland, are dispersed, by the operation of wink- 

 ing, over the front of the eye, for the purpose of preserving the 

 transparent part of it humid and bright ; and as they make their 

 way to the bottom of the eye by gravitation, they naturally run 

 along the triangular canal to the inner canthus, and become di- 

 rected by the caruncula into the puncta, which, it is thought, 

 imbibe them by some peculiar vital action, and not by any sup- 

 posed capillary attraction*. When the tears overflow their 

 proper channels, and trickle down the side of the face, the eye is 

 said to be watery : this is commonly the condition of the organ 

 on the approach of ophthalmia; it arises also in states of common 



* Whether do the puncta transmit the tears into that which leads to the 

 lachrymal bag, in consequence of the fluid being impelled by force, or 

 whether they have the power of absorbing the tears ? We have reason to 

 believe that the orbicularis does, jointly with other powers, impel the tears 

 onward. A person having a redundancy of tears, shuts his eyes, and 

 gently impresses the fluid with that muscle, in order to get rid of them. 

 But yet there is reason to believe that there is an absorbing power. Dr. 

 Fullerton was convinced that they (the puncta) had this power ; and I think 

 it reasonable myself. — Abernethy's Lectures. 



