THE SENSE OF SIGHT. 675 



substances gives rise to the sensation of yellow, that of red and blue to the 

 sensation of purple, and that of blue and green to the sensation of blue-green. 

 Simultaneous stimulation of all three substances of a certain area produces 

 the sensation of white. According to this theory, complementary colors 

 are any two which together excite all three substances. Color-blindness 

 is explained by this theory, on the assumption that two of the photo-chemical 

 substances have become similar or equal in composition to each other. 



The theory oj Hering, brought forward in 1874, has the underlying 

 assumption that the process of restitution in a nerve-element is capable of 

 exciting a sensation. This theory asserts that there are three visual sub- 

 stances in the retina a white-black, a red-green, and a yellow-blue visual 

 substance. A destructive process in the white-black substance, such as is 

 induced not only by white light, but also by any other simple or mixed color, 

 produces the sensation of white, while the process of restitution or assimila- 

 tion in this substance produces the sensation of black. Similarly, red light 

 produces dissimilation 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 dissimilation and assimila- 

 tion in the same visual substance antagonize each other, and consequently 

 produce no color-sensation 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 produce only 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 lachrymal 

 glands, and tears. 



The eyelids consist of a central framework of connective tissue support- 

 ing muscle tissue (the orbicularis palpebrarum muscle) and glands, and 

 covered externally by skin and internally by a modified skin, the conjunctiva. 

 The free border of each lid is strengthened by a semilunar plate of dense 

 fibrous tissue, the tarsus. The cutaneous edge of the lid is bordered with 

 short stiff hairs. At the inner extremity each eyelid presents a small opening, 

 the punctum lacrimale, the beginning of the lachrymal duct. The two ducts 

 after uniting open into the nasal duct. 



The Meibomian glands are modified sebaceous glands imbedded in the 

 posterior portion of the lids (Fig. 335). Their ducts open on the free border 

 of the lid. These glands secrete an oleaginous material resembling sebace- 

 ous matter, which accumulates along the margin of the lid and prevents the 

 tears from flowing down the cheek. 



The lachrymal gland is situated at the upper and outer part of the orbit 

 cavity. It consists of a series of compound tubules lined by epithelium. 



