Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 244 AYE*. CM 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 (tig. 654) of the Bird has an unusual form, in that the hind part on which the retina is spread is a segment of a mucn
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 244 AYE*. CM 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 (tig. 654) of the Bird has an unusual form, in that the hind part on which the retina is spread is a segment of a mucn 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 nocttirnal birds of prey, and least in the aquatic Birds in which the axis of the eye is shorter. There i^ ahvavs 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 Apteryv'^ 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 falciform process of the piscine and reptilian eye. The 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, w
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