EYE OF THE ENGLISH SPARROW 361 



insertions in the two lids. At their proximal end they are nar- 

 row, but as they extend outward they become gradually broader 

 until they reach their insertion. In the other dimension they 

 are thin and ribbon-like. The lower muscle is larger than the 

 upper. The insertion of the inferior palpebral muscle is at the 

 lower anterior surface of the tarsal-like plate. This muscle is 

 somewhat larger about midway between its two attachments. 

 The superior palpebral is more slender and has more tendon and 

 less muscle tissue in its distal portion than the inferior. Its 

 insertion is well down toward the marginal folds of the upper lid. 

 No plate-like structure is found in this lid. 



The orbicularis palpebrarum, as already mentioned, is com- 

 posed of non-striated fibers, which are arranged approximately 

 parallel to the margins of the lids. It functions in closing the lids. 



One marked difference is noticed in the rapidity with which 

 the lids are opened and closed.' They open rapidly and close 

 very slowly. This is due to the structure of the muscles in- 

 volved in these two movements. The quick opening of the eye 

 is often of urgent need to the bird in order to save its life from 

 some danger while the closing is not so vital. This rapid open- 

 ing can only be accomplished by striated muscle fibers. 



When at rest the nictitating membrane, or third eyelid, is 

 scarcely visible in the interpalpebral space. Even when it is 

 pulled over the front of the eye, a correct idea of its size and 

 shape is not obtained, as only a portion of it is exposed to view. 

 When the lower lid is dissected loose and turned down, and the 

 upper part of the eye is exposed, the full extent of the mem- 

 brarfe is seen. Figure 5 shows it in the relaxed condition. It 

 forms, with the eyeball, a pouch or bag at its lower anterior por- 

 tion, into which the duct from Harder's gland opens. The 

 secretion from this gland is thus poured under the nictitating 

 membrane and directly on the front of the eye. 



Numerous folds occur on the surface of the nictitating mem- 

 brane. Sections show these to be largely superficial and con- 

 fined almost wholly to the epithelial layer of the conjunctiva 

 which envelops the membrane. They largely disappear when 

 the membrane is extended over the eye. 



