VISUAL OEGAN OF VEETEBRATA. 531 



of smootli muscular fibres, is provided witli a swelling (campanula 

 Halleri), wliicli is attached to tlie hinder part of the capsule of the 

 lens. These processes are also found, in a somewhat modified condi- 

 tion, in the eyes of Reptiles and Birds. In the Saurii there is a 

 curved and thickened fold which extends to the margin of the 

 capsule of the lens, at the side of which there may be several other 

 folds (Fig. 297, j>). This structure is feebly developed in the eye of 

 the Crocodiliui. In Birds it is remarkable for the increase in the 

 number of its folds, and is distinguished as the " pecten^' (Fig. 298, j7). 

 In many Natatores and Grallatores it reaches as far as the capsule 

 of the lens. In the Struthiones the end of the pecten is widened out 

 into a pouch (marsupium). In Apteryx, as in the Mammalia, it is 

 absent. The point, at which the optic nerve enters, varies with the 

 characters of this process, for when it is Avidened out at its base the 

 nerve is placed more to the side of the eye. 



With regard to the lens, the difference in its form, in accordance 

 with the surrounding media, is a noteworthy point. In Fishes it is 

 very large, and quite spherical, as it is also in the Amphibia, and in 

 aquatic Mammalia; while in others, and in Birds and Reptiles, we 

 meet with more flattened forms of lenses; the amount of flattening 

 is, of course, very various. The internal cavity of the eye is divided 

 into an anterior and a posterior space by the attachment of the lens 

 to the ciliary portion of the choroid. The vitreous body fills the 

 hinder one; the anterior one, which lies between the anterior 

 surface of the lens and the cornea, is often but a very small portion 

 of the whole eye. It is filled by the aqueous humour. 



§ 399. 



Accessory organs, which partly serve to move, and partly to 

 protect the bulb, are connected with the eye. The movements of 

 the eye are generally effected by six muscles, of which four are 

 straight and two oblique. They are atrophied in the Myxinoidea. 

 In many Teleostei the straight ones are embedded in a canal at the 

 base of the skull ; this is in adaptation to their length, which again 

 is due to the large size of the bulb. They take their origin from a 

 point which is placed some way behind that at which the optic nerve 

 passes out; it is in the higher divisions only that they acquire 

 relations to this point. In the Amj^hibia and Reptilia there is 

 a retractor of the bulb, in addition to the four straight muscles. 

 This is retained also by most of the Mammalia, and breaks up into 

 several portions (in Carnivora into four), which pass to the bulb from 

 the point at which the optic nerve enters the orbit. In the Mam- 

 malia the superior oblique, which, like the inferior oblique, ordi- 

 narily arises from the median wall of the orbit, is altered in character. 

 It has the same origin as the straight muscles of the eye, and the 

 tendon of insertion passes to the bulb through a pulley, and at an 

 angle. 



Of the pi'otective organs of the eye, the eyelids are folds of the 



2 M 2 



