xvii THE SENSE ORGANS 325 



outer fibrous membrane, and an inner layer of hyaline car- 

 tilage ; on the side next to the cranium it is perforated by an 

 aperture, the optic foramen, through which the optic nerve 

 enters the eye. 



At its outer margin the cornea is continuous with a mem- 

 brane, the conjunctiva, which passes backward a short dis- 

 tance (about 2 mm.) over the sclerotic, and is then reflected 

 over the inner surface of the upper and lower eyelids. The 

 upper eyelid is a narrow, thick fold extending from the upper 

 edge of the orbit. It has no power of independent motion, 

 but may be raised or lowered by the movements of the eye- 

 ball. What appears as the lower lid is really made up of 

 two elements, the lower lid proper, and the nictitating mem- 

 brane. In most animals these folds are quite distinct, but 

 in the frog the nictitating membrane is continuous with the 

 upper portion of what corresponds to the true lower eyelid 

 of other forms. The lower lid consists of a lower thick por- 

 tion and an upper thinner part, which is folded in behind the 

 former when the eye is uncovered. The nictitating mem- 

 brane is a thin semilunar fold somewhat more transparent 

 than the upper portion of the lower eyelid, from which it is 

 separated by a slight furrow. At the anterior and posterior 

 ends of its slightly thickened upper margin it is attached to 

 the two ends of a tendon which passes behind the eyeball, 

 thus encircling the greater part of the eye. The closing of 

 the eye of the frog does not occur through the independent 

 movements of the eyelids as it does in ourselves, but is a 

 consequence of the retraction of the eye within the orbit. 

 If one watches the closing of the eye of the frog, it will be 

 seen that every time the lids are drawn together the eye 

 sinks into the head. The withdrawal of the eye by produc- 

 ing a tension on the tendon of the nictitating membrane 

 causes the latter to be pulled up, although one might easily 



