542 STUDIES IN ADVANCED PHYSIOLOGY. 



2. The Eyelids. Each eye is provided with two main 

 eyelids and the remnant of a third. Of these the upper 

 eyelid is the one which has the greatest range of motion, 

 and by the movements of which the lachrymal secretion is 

 being continually washed across the globe. In addition to 

 distributing the tears it serves purposes of protection against 

 foreign particles and for the exclusion of light when that 

 is too intense, or when, as in sleep, it is to be excluded 

 altogether. The functions of the upper eyelid apply to a 

 certain extent equally to the lower eyelid. 



At the edge of these eyelids are found the eyelashes, 

 rows of hairs serving to prevent foreign particles from get- 

 ting into the eye, and possibly shading the cornea. Be- 

 tween these hairs open the ducts of specially developed 

 sebaceous glands, known as the Meibomian glands, which 

 secrete a fatty substance specially evident in certain cases 

 of inflammation, where this secretion becomes so excessive 

 as to stick the eyelids together. This secretion serves not 

 only to keep the edge of the eyelids pliable, but prevents the 

 tears from running over the eyelids, it being a general phy- 

 sical fact that water does not readily flow over an oily 

 surface. Of course in the act of crying the amount of the 

 secretion increases so much that this oily edge no longer 

 serves as a sufficient barrier, and so the tears stream down 

 the cheeks. 



Lining the under sides of the eyelids both upper and 

 lower and reflected back over the front of the cornea is the 

 conjunctiva. Few membranes in the body, especially those 

 exposed to the exterior, are so richly supplied with nerves 

 and so sensitive to all foreign objects. The explanation of 

 placing such a very sensitive membrane under the eyelids 

 and over the cornea lies in the fact that it prevents us from 

 becoming careless about injurious particles reaching the 

 eye, and so gives iis no rest until the irritating object is 

 removed. 



The lifting of the upper eyelid is effected by muscles 

 running down into the eyelid. The closing of the eye is 



