252 SENSE OF SIGHT. 



and elaboration has been going on, is irresistible ; but there is no such 

 action in the case in question. The tears in the lachrymal ducts and 

 ductus ad nasum are identical with those spread upon the surface of 

 the eye. This is one of the few cases in the human body, which admit 

 of satisfactory explanation on the physical principles of capillary 

 attraction. In vegetables, the whole of the circulation of their juices 

 has been thus accounted for. If we twist together several threads of 

 yarn; moisten them; and put one extremity of the roll into a vessel of 

 water, allowing the other to hang down on the outside and to dip into 

 an empty vessel placed below it, we find, that the whole of the fluid 

 in the first vessel is in a short time transferred to the second. If, 

 again, we take a small capillary tube, less than the twentieth part of 

 an inch in diameter, and place it so as to touch the surface of water, 

 we find, that the water rises in it to a height, which is greater, the 

 smaller the bore of the tube. If the diameter of the tube be the fiftieth 

 part of an inch, the water will rise to the height of two inches and a 

 half; if the one hundredth part of an inch, to five inches; if the two 

 hundredth part of an inch, to ten inches ; and so on. Now, the punc- 

 tum lachrymale is, in our view of the subject, the open extremity of a 

 capillary tube, which receives the fluid of the lachrymal gland and con- 

 veys it to the nose, the punctum being properly directed towards the 

 eyeball by the tensor tarsi muscle of Homer, and the inspiratory 

 movements drawing it down the ductus ad nasum. 



Lastly, the tunica conjunctiva is another part of the guardian ap- 

 paratus of the eye. It secretes a fluid, which readily mixes with the 

 tears, and appears to have similar uses. Like mucous membranes in 

 general, it absorbs; and, in this way, a part of the lachrymal secretion 

 is removed from its surface. An animal, for the same reason, can be 

 readily poisoned by applying hydrocyanic acid to it. As the conjunc- 

 tiva lines the eyelids, and is reflected over the globe, it supports the 

 friction, when the eyeball or eyelids are moved; but, being highly 

 polished and always moist, this is insignificant. 



The extreme sensibility of the outer part of the eye appertains to 

 the tunica conjunctiva, and is dependent on the ophthalmic branch of 

 the fifth pair. When this nerve was divided in a living animal, M. 

 Magendie 1 found, that the membrane became entirely insensible to 

 every kind of contact, even of substances that destroyed it chemically. 

 In his experiments on this subject, he arrived at singular results, re- 

 garding the influence of the fifth nerve on the nutrition of the eye. 

 When the trunk of the nerve was divided within the cranium a little 

 after its passage over the petrous portion of the temporal bone, the 

 cornea was found, about twenty-four hours afterwards, to become 

 troubled; and a large spot to form upon it. In the course of from 

 forty-eight to sixty hours, the part was completely opaque; and the 

 conjunctiva, as well as the iris, in a state of inflammation ; a turbid 

 fluid was thrown out into the inner chamber, and false membranes pro- 

 ceeded from the interior surface of the iris. The crystalline and 

 vitreous humours now began to lose their transparency; and, in the 



1 Precis Elementaire, ii. 494. 



