222 PHILOSOPHY OF ZOOLOGF. 



fourth toe is wanting in some species, in others, it consists 

 of one or more joints. Whatever number of joints the 

 fourth or hind toe possesses, the inner toe has one, the 

 middle two, and the outer three joints more, in the greater 

 number of birds. 



Birds, in accomplishing progressive motion on land, make 

 use of their posterior extremities, in walking, hopping, or 

 running. In flying, the wings alone produce the motion, 

 while the tail regulates the course. In swimming on the 

 surface of the water, the legs are exclusively employed, 

 but when motion is accomplished beneath the surface, the 

 wings are then chiefly in exercise. 



The third eye-lid, or membrana nictitans, is here so 

 perfect, as (when drawn out) to cover the whole eye-ball. 

 The external margin of the sclerotic coat is split into two 

 laminae, between which, a circle of osseous plates is inter- 

 posed. The iris is variously coloured, according to the 

 species, and is frequently used as a discriminating charac- 

 ter. It is subject, however, to change with age. The op- 

 tic nerve terminates in a white line, from the sides and ends 

 of which the retina is produced. The marsupium ni-, 

 grum or pecten^ is suspended the whole length of this line^ 

 penetrates the vitreous humour, and reaches nearly to the 

 lens. It is composed of vascular folds covered with a black 

 pigment. Its use is unknown. 



There is no external ear, although the feathers are so 

 arranged as to supply the place of a concha. In many 

 species these feathers are peculiar in their form and even 

 colouring. 



The nostrils exhibit many important characters. These 

 are chiefly derived from their form, their position in the 

 bill, and their relation to the feathers at the base. 



The digestive organs exhibit many varieties of form. 

 There is, properly speaking, no pharynx, no uvtila nor 



