372 SENSITIVE SYSTEM. 



tion appears to be one for which the Hds are by no means well 

 adapted. If a foreign body happens to lodge in the eye of a man, 

 the lids frequently rather tend to retain than to dislodge it; and, 

 were it not for his hands, it would occasionally remain there and 

 create dreadful irritation : so it would be with a horse, were he not 

 provided with a membrana nictitans, which here serves him in 

 the place of hands in performing an office to which the lids in a 

 general way are of themselves inadequate. Why was the monkey 

 not furnished with a membrana nictitans ? Because he can 

 make use of his fore paws after the manner of hands. The eye 

 of a man therefore, as well as that of a monkey, is, abstracted/i/ 

 considered, an imperfect organ compared with the eye of a horse. 

 What supervenes upon the admission of any insect or foreign 

 substance into the eye (a rare occurrence among horses) is this, — 

 the convulsive twinkle consequent upon the accident commonly 

 carries the foreign body underneath the lid, where it soon creates 

 irritation and pain ; tears are then shed in profusion, and the 

 membrana nictitans during the time is repeatedly slid across the 

 eye : in one of these efforts, the irritating substance is dislodged 

 from its place, and in the next, probably withdrawn upon the con- 

 vex part of the membrane, from which it becomes subsequently 

 washed off by the tears. — But (since this part has no muscle 

 whatever in its composition, nor any attached to it) how is it put 

 into motion ? In this manner — The space at the bottom of the 

 orbit unoccupied by the globe is completely filled with adeps; the 

 globe consequently cannot be retracted without displacing some 

 of this fat — that being liquid in the living subject, and on that 

 account incompressible ; and as the globe, when it is retracted, 

 is drawn with an inclination inward, it is the fat next the nose, 

 or that in which the membrana is cushioned, that must give way. 

 It is quite evident what the result of this must be. The fat is 

 forced against the membrana, and that being moveable becomes 

 projected (guided by the eyelids and confined by the conjunctiva) 

 over the transparent cornea ; which it more effectually covers, 

 from the circumstance of the globe being at the same instant 

 turned inward. The retracting muscles relaxing, the adeps, from 

 its own inherent elasticity, recedes back into its place, drawing 

 the membrana nictitans along with it. 



The Lachri/rnal Gland. 



To obtain a full view of this part iii situ, the orbital arch 

 must be removed. Underneath this process of bone we find the 

 gland, covered by the common aponeurotic lining of the orbit, 

 supported by the eyeball, cusiiioned upon the levator palpebrse, 

 and enveloped in fat and cellular membrane, by which it is con- 



