SENSITIVE SYSTEM. 371 



sleep with their eyes open. Even the act of winking has a re- 

 freshing eftect, from intercepting the hght even but for an instant. 



Mernbrana Nictitans. 



The membrana nictitans (by farriers called the haw) is 

 a concavo-convex cartilaginous body, lodged behind the inferior 

 canthus, between the eyeball and side of the orbit. In a perfectly 

 healthy condition of these parts only the cuticular margin of this 

 substance is naturally visible, and that preserves the line of the 

 transparent cornea : but in a morbid or preternatural state of 

 sensibility it is protruded forward, and encroaches more or less 

 upon the transparent part of the eye. In a detached state, it 

 approaches in figure to an extended triangle, of which the short 

 side is turned forwards, the lengthened angle backwards. The 

 anterior part is thin, very elastic, and is bounded by a crescentic 

 edge, terminated by two salient angles, by which it is shaped to 

 the inward third of the circumference of the cornea ; it increases 

 in substance but grows narrow posteriorly, and there ends in a 

 thick, obtuse, conical point, which is sunk into the adipose sub- 

 stance at the bottom of the orbit : thus it covers the inner and 

 inferior sides of the globe — about one third of its entire superficies. 

 Inwardly, it is evenly concave, to adapt itself to the globe; out- 

 wardly, it is unevenly convex, and clothed with adeps. With 

 the exception of the posterior end, this body is enveloped in con- 

 junctiva, which, though it adheres closely to the thin portion, so 

 loosely invests the thick part, that very free and extensive motion 

 is admitted. The crescentic margin is edged with cuticle, com- 

 monly black, sometimes pied ; and this is the only part visible 

 externally. Though it has got the name of a membrana, it has a 

 thin piece of cartilage for its base, to which it owes its shape : 

 this cartilage is thin and pliant, becomes thicker and more resist- 

 ing posteriorly, and is interposed between two tough brownish 

 substances of a ligamentous nature, the outer of which is double 

 the thickness of the inner. 



Use. — Writers on comparative anatomy treat of this body as a 

 third eyelid. In birds it certainly answers this purpose in the 

 ordinary way, and in some quadrupeds too; but it appears to 

 have been given also for a purpose which the lids alone could not 

 accomplish, or at least not perform with the same ease and effect. 

 If one makes a feint to strike the eye of a horse, one perceives 

 that the convulsive twinkle of the lids is accompanied by the 

 momentary sliding of the membrana nictitans over the eyeball, 

 the same as a man would oppose his arm (or a shield if he had 

 one) to ward off the blow : in this case it affords greater protec- 

 tion to the organ than what the lids offer. But its chief opera- 



