VETERINARY OPHTHALMOLOGY. 29 



insinuated between ail the muscles of the ' eye. The 

 movements of the haw are mechanical, and no muscle 

 directly causes them. When the eye is in repose but 

 a small fold ,of conjunctiva is seen ; the rest is in its 

 fibrous case. When the eye is withdravi^n into the 

 orbit by contraction of the recti muscles, the globe 

 compresses the fatty cushion belonging to the carti- 

 lage; this cushion, pressing outwards, pushes the 

 membrana before it, and the latter then entirely con- 

 ceals the fi-ont of the eye. This movement is instanta- 

 neous, but it may be momentarily fixed by pressing 

 gently on the eye when the animal retracts it within 

 the orbital cavity. The use of the membrana is, as will 

 be seen from the above, to maintain the healthy con- 

 dition of the eye by removing any matters that have 

 escaped the eyelids; and what clearly demonstrates 

 this function is the inverse relation that always exists 

 between the development of this body and the facility 

 with which animals can rub their eyes with their ante- 

 rior limbs ; so it is that, with the horse and the ox, whose 

 thoracic member cannot be applied to this purpose, 

 the membrana is very highly developed, and in the 

 dog, which may use its paw to some extent when it 

 requii-es to brush its eye, it is smaller ; in the cat it is 

 still less, while in the monkey and in mankind, whose 

 hands are perfect, it is rudimentary. In tetanus, the 

 membrana nictatans often remains permanently over 

 the eye in consequence of the continued contraction of 



