THE MUSCLES OF THE EYE AND EAR. 57 



cartilaginous plate, here found between the conjunctival 

 membrane and the ligamentous layer. It is of interest 

 to know that in the Strigidce and Caprimulgi the 

 eyes are closed principally through the depression of 

 the upper lids. 



In common with most other birds, the Eaven possesses 

 a well- developed memhrana nictitans, which is here 

 of a pearly w^hite colour, and consequently opaque. As 

 it closes, it passes obliquely across the eyeball, from its 

 resting-place in the supero-internal corner, to the one 

 opposite, a movement effected by two special muscles, 

 the next to be described (Fig. 21). 



40. TJie quadrat Its nictitantis hnds its origin upon the 

 sclerotic at the upper part of the ball, behind. From this 

 point the fibres converge as they pass towards the optic 

 nerve, and are inserted in an aponeurosis which sheathes 

 the latter above. It is evident that in this course the 

 fibres of the quadratus pass downwards, inwards, and 

 backwards, interfering in no way with the action of the 

 other muscles, have no fixed bony insertion, and do not 

 stand in the way of the passage of the rays of light. 



41. The lyyvamidaUs nictitantis, the remaining muscle 

 acting upon the membrana nictitans, occurs in the Raven 

 precisely as we find it in the Goose, and it has been 

 so well described by Owen for that bird, that I quote his 

 words. He says the " p^/7'am^G?aZ^s fiictitantis (Fig. 21, h, 

 of the present work) arises from the low^er and nasal side of 

 the eyeball : its fibres converge towards the upper part of 

 the optic nerve, and terminate in a small round tendon 

 which glides through the pulley at the free margin of 

 the quadratus ; thus, winding over the nerve, it passes 

 down to be inserted into the lower part of the margin of 

 the third eyelid. By the simultaneous action of the 

 two muscles, that nictitating lid is drawn outward and 



