436 



AVES. 



enlargement of the lumbar region so as to form a second sinus 

 rhomboidalis (Fig. 240). The cranial nerves are all separate 

 and their distribution is essentially the same 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 moveable, especially 

 the lower lid and the transparent nictitating membrane, which 

 is drawn over the eye by a peculiar muscular apparatus. The 

 eyeball (Fig. 241) 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 for- 

 wards. 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 short. There is always a 

 bony sclerotic ring behind the edge 

 of the cornea. The cornea is 

 strongly arched, while the anterior 

 surface of the lens only possesses 

 a considerable convexity in noc- 

 turnal birds. The pecten ' (wanting 

 only in Apteryx) is a peculiar 

 structure of the avine eye. It 

 consists of a process of the cho- 



roid, which traverses the retina near the optic nerve and 

 passes obliquely through the vitreous humour to the lens. It 

 corresponds to the falciform process of the piscine and reptilian 

 eye. The avine eye is characterised not only by the sharpness 

 of vision consequent on the large size and complicated structure 

 of the retina (many birds possess two foveae centrales in each 

 eye) : but also by the highly-developed power of accommodation, 

 which is principally due to the muscle of the so-called ciliary 

 ligament (Crampton's muscle), and also to the great mobility of 

 the muscular iris, which possesses both sphincter and dilator 

 muscles. The sphincter is supplied by the third nerve and is 



Rt 



FIG. 241. Eye of a nocturnal bird 

 of prey (after Wiedersheim) . 

 CM ciliary muscle ; Co cornea ; 

 L lens : N.o optic nerve ; P pec 

 ten ; Rt retina ; Sc ossifications of 

 the sclerotic. 



