THE SENSE OF SIGHT. 635 



by any other simple or mixed color, produces the sensation of white, 

 while the process of restitution or assimilation in this substance pro- 

 duces the sensation of black. Similarly, red light produces dis- 

 assimilation or decomposition in the red-green substance, and this, 

 again, the sensation of red. Green light, however, favors the process 

 of restitution or assimilation in the red-green substances, and thus 

 gives rise to the sensation of green. In the same way the sensation 

 of yellow has its cause in the decomposition of yellow-blue substance 

 induced by yellow light, while the sensation of blue is produced 

 by an assimilative process in the same substance. Simultaneous 

 processes of disassimilation and assimilation in the same visual sub- 

 stance antagonize each other, and consequently produce no color-sen- 



FIG. 308. THE LACRIMAL AND MEIBOMIAN GLANDS, AND ADJACENT ORGANS OF 

 THE EYE. i, i. Inner wall of orbit. 2, 2. Inner portion of orbicularis palpe- 

 brarum. 3, 3. Attachment to circumference of base of orbit. 4. Orifice for 

 transmission of nasal artery. 5. Muscle of Horner (tensor tarsi). 6, 6. Mei- 

 bomian glands. 7, 7. Orbital portion of lacrimal gland. 8, 9, 10. Palpebral 

 portion, n, n. Mouths of excretory ducts. 12, 13. Lacrimal puncta. (Sappey.) 



sation by means of this substance, but only the sensation of white, by 

 reason of decomposition, by both colors, in the white-black substance. 

 Thus, yellow and blue, impinging on the same retinal area, have no 

 effect on the yellow-blue substance, because they are antagonistic in 

 their action on this substance, but only produce the sensation of white, 

 as both yellow and blue decompose the white-black material. Color- 

 blindness is explained by the assumption of the absence of either the 

 red-green or the yellow-blue visual substance in the retina. 



Accessory Structures. The eyeball is protected anteriorly by 

 the eyelids and their associated structures, the Meibomian glands, 

 the lacrimal glands, and tears. 



