260 ANATOMY AND PHYSIOLOGY FOR NURSES 



The nasal duct, a part of the lacrimal apparatus, is 

 a short canal, about f inch in length, directed downward, 

 backward, and a little outward to the inferior mea- 

 tus of the nose, where it terminates by an expanded ori- 

 fice. It is provided with a mucous membrane, continu- 

 ous with the conjunctiva above and with the membrane 

 of the nose below. Obstruction of this duct from inflam- 

 mation and suppuration leads to the formation of a 

 lacrimal fistula. The nerves and vessels are derived 

 from the ophthalmic branches. 



PHYSIOLOGY OF VISION 



After reviewing the anatomic relation of sight, we will 

 now take up a few essential points regarding the physio- 

 logic action of the various structures. 



It is by the sense of sight that we receive the impres- 

 sion of light and color, as well as the ideas of construction 

 of materials, establishing certain fancies, likes, and dis- 

 likes of the external surroundings. 



The eyeball is constructed like a photographer's camera. 

 The sclerotic and choroid coats correspond to the inside 

 walls of the chamber, while by the refractive power of 

 the several parts, as the cornea, aqueous humor, crystal- 

 line lens, and vitreous humor, the image is formed. The 

 retina represents the sensitive plate on which the image is 

 received. The iris regulates the rays of light, acting as 

 a diaphragm to cut out certain portions of light that are 

 not required. The ciliary muscles adjust the shape of 

 the crystalline lens so as to focus images upon the 

 retina. 



The function of the crystalline lens is to focus the rays 

 of light with the formation on the retina of the image, 

 corresponding with the object from which the light pro- 

 ceeds. By accommodation is meant the power of adjusting 

 the eye to certain distances. A normal eye does not 



