SEC. Ill EXTERIOR PARTS OF BIRDS 135 



work like men hoisting sails from the deck of a vessel ; and thus, 

 like a ship's cargo, a bird's chief weight is kept below the centre of 

 motion. Top-heaviness is further obviated by the way in which 

 birds with a long heavy neck and head draw these parts in upon 

 the breast, and extend the legs behind, as is well shown by the 

 attitude of a heron flying. The nice adjustment of balance by the 

 variable extension of the head and feet is exactly like that produced 

 in weighing by shifting a weight along the arm of a steelyard ; 

 and together with the slinging of the chief weight under the wings 

 instead of over or even between them, enables a bird to easily keep 

 right side up in flight. The 



ExteriOF of a Bird is divided for purposes of description into 

 S6«« parts : — 1. The head (Lat. cwpui); 2. The neck (Lat. collwm); 

 3. The body proper, or trunk (Lat. truncus) ; 4. The bill or beak 

 (Lat. rostrum) ; 5. The wings (Lat. pi. alee) ; 6. The tail (Lat. cauda) ; 

 7. The feet (Lat. pi. pedes). Of these, 1, 2, 3, the head, neck, and 

 trunk, are collectively termed the body (Lat. corpus), in distinction 

 from 4, 5, 6, 7, which are the members (Lat. pi. memh-a). The 

 wings and feet are of course double or paired parts. The bill is 

 strictly but a part of the head ; but its manifold uses as an organ 

 of prehension make it functionally a hand, and therefore one of the 

 " members." The 



Head has the general shape of a four-sided pyramid ; of which 

 the base is applied to the end of the neck, therefore not appearing 

 from the exterior, and the apex of which is frustrated at the base 

 of the bill. The uppermost side is more or less convex or vaulted, 

 sloping in every direction ; the under side is flattish and horizontal ; 

 the lateral surfaces are flattish and vertical ; all similarly taper 

 forward. The departures from any such typical shape are endless 

 in degree and variable in kind, giving rise to numerous general 

 descriptive terms, such as "head flattened," "head globular," but 

 not susceptible of exact definition. The head is moulded, of course, 

 upon the skull, corresponding in a general way to the brain-cavity 

 of the cranium proper, both in size and shape ; but it differs in 

 several particulars. In the first place, there is the scaffolding of 

 the jaws ; secondly, large excavations to receive the eyeballs, and 

 smaller ones for the ear-parts ; thirdly, muscular masses overlying 

 the bone ; and lastly, in some birds, large hollow spaces in the bone 

 between the inner and outer tables or plates of the cranial walls. 

 Each side of the head presents two openings for the eye (Lat. oculus) 

 and ear (Lat. auris), the position of which is variable, both absolutely 

 and in relation to each other. But in the vast majority of birds, 

 the eye is strictly lateral in situation, and near the middle of the 

 side of the head ; while the ear is behind and a little below the 

 eye, near the articulation of the lower jaw. But the shape of the 



