244 



AVES. 



as in the Mammalia. The spinal cord reaches almost to the end of 

 the neural canal of the vertebral column. 



Sense Organs. The eyes always attain a considerable size and a 

 high development. The eyelids are always movable, especially the 

 lower lid and the transparent nictitating membrane, which is drawn 

 over the eye by a peculiar muscular apparatus. The eyeball (fig. 

 G54) of the Bird has an unusual form, in that the hind part on 

 which the retina is spread is a segment of a much larger sphere than 

 is the small anterior part. The two parts are connected by a median 

 portion, which has the shape of a short truncated cone, with the 

 smallest end directed forwards. This form of the eyeball is most 



marked in the nocturnal birds of prey, 

 and least in the aquatic Birds in which 

 the axis of the eye is shorter. There is 

 always a bony sclerotic ring behind the 

 edge of the cornea. The cornea, except 

 in the swimming Birds, is strongly 

 arched, while the anterior surface of 

 the lens in the nocturnal Birds alone 

 possesses a considerable convexity. The 

 pecten (wanting only in Apteryx) is a 

 peculiar structure of the avian eye. It 

 consists of a process of the choroid, which 

 traverses the retina and passes obliquely 

 through the vitreous humour to the lens. 



_ . 



It corresponds to the ialciiorm process 

 o f fa e piscine and reptilian eye. The 



r \ 



avian eye is characterised not only by 

 the sharpness of vision consequent on the large size and complicated 

 structure of the retina, but also by the highly-developed power of 

 accommodation, which is principally due to the muscle of the so- 

 called ciliary ligament (Krampton's muscle), and also to the great 

 mobility of the muscular iris (dilatation and contraction of the pupil). 

 The auditory organ (fig. 578 //.) is enclosed by spongy masses of 

 bone. It possesses three large semicircular canals and a dilated 

 cochlea (lagena). The vestibule, which on account of its small size 

 may also be regarded as the lower dilated part of the cochlea, has 

 two openings : the foramen ovale which is closed by the terminal 

 piece (operculum) of the columella and looks into the tympanic 

 cavity, and a second more rounded opening, the foramen rotundum, 

 which is closed by membrane. In addition to the labyrinth there is 



Rt 



FIG. 654.-Eye of a nocturnal bird 

 of prey (after wiedersheim). Co, 



cornea ; L, lens ; Et, retina ; P, 



pecten; NO, optic nerve; So, 

 ossifications of the sclerotic ; CM. 



ciliary muscle. 



