i8 ANIMAL BIOLOGY. [Parti. 



products. The third median aperture is at the summit of a little 

 papilla above the tail. It is the orifice of the duct of the uro- 

 pygial oil-gland. The lateral apertures are the anterior nares, just 

 in front of the cere, and the ear-openings at the side of the head. 

 The fore-limb is divided into brachium, antibrachium, and manus; 

 but the manus is not divided into separate digits, such of the 

 fingers as exist being united into a continuous mass, while the 

 rudimentary pollex (thumb) forms an insignificant projection. 

 The quill feathers attached to the manus are called primaries \ 

 those attached to the antibrachium secondaries. The second and 

 third primaries are longer than the first, and form the tip of the 

 wing. The pollex bears a little group of feathers called the 

 bastard wing. The quills of both primaries and secondaries 

 are overlapped above and below by wing coverts. On the dorsal 

 surface of the brachium is the humeral feather tract, and on that 

 of the antibrachium the alar tract. 



Thus the fore-limb is especially developed for flight. It is a 

 homologous organ with that of the rabbit, but not an analogous 

 organ. Homologous organs are those that are built upon the same 

 plan; analogous organs those that perform the same function. The 

 hind-limbs of the rabbit and the pigeon are both homologous and 

 analogous, but the fore-limbs are homologous but not analogous ; 

 while the wings of a bird and of a butterfly are analogous, for 

 they perform the same function, but not homologous, for they 

 have no community of plan. 



The hind-limb reminds us of that of the frog, in that it has 

 four divisions. The full meaning of these divisions will only 

 become clear when we study the osteology of the hind-limb. 

 For the present we may regard them as femur, crus, and pes ; 

 the latter having an undivided portion, and a divided portion 

 with four digits. The hallux, or great toe, is directed backwards, 

 the other three toes forwards ; the fifth is absent. There is a 

 well-developed femoral tract of feathers on the femur, and a less 

 developed crural tract on the crus. The pes throughout is devoid 

 of feathers, 1 but has instead well-marked red scales. 



The position of the fore-limb during flight is nearly that which 

 1 Not so iii some artificially modified pigeons. 



