EXTERNAL PARTS OF BIRDS. — TOPOGRAPHY. 97 



■bill, and contracts behind toward tlie middle of the neck, in consequence of diminution in 

 bulk of the muscles by which it is slung on the neck ; which last is somewhat contracted or 

 hour-glass shaped near the middle, swelling where it is slung to the body. The body is largest 

 in front and tapers to the tail. 



The Centre of Gravity is admirably preserved beneath the centre of the body, and op- 

 posite the points where it is supported by the wings. The enormous breast- muscles of a bird 

 axe among its heaviest parts, sometiuies weighing, to speak roundly, as much as one-sixth of 

 the whole bird. Now these are they that effect all the movements of the wings at the shoulder- 

 joints, lifting as well as lowering the wings. Did these pectoral muscles pull straight, the 

 lifters would have to be above the slioulder-joiut ; but they all lie below it, and the lifters 

 accomplish their office by running through pulleys to change the line of their traction. They 

 work like men hoisting sails from the deck of a vessel ; and thus, like a ship's cargo, a bird's 

 <;hief 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 adjust- 

 ment 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 steel-yard; 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 riglit side up in tlight. 



The Exterior of a Bird is divided for purposes of description into seven parts : — 

 1. Head (hat. caput) ; 2. Neck (hat. collum) ; 3. Body proper, or trunk (Lat. truncus) ; 4. 

 Bill or beak (Lat. rostrum) ; 5. Wings (Lat. pi. alee) ; 6. Tail (Lat. Cauda); 7. Feet (Lat. pi. 

 pedes). Of these, 1, 2, 3, head, neck, and trunk, are collectively termed body (Lat. corjms). 

 in distinction from 4, 5, 6, 7, which are members (Lat. membra). 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 frus- 

 trated 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 sliape ; but it differs in several particulars. In the first place, 

 there is the scaff"olding of the jaws; secondly, large excavations to receive the eye-balls, and 

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

 the bone ; and lastly, in some birds, large liollow spaces in bone between the inner and outer 

 tables or plates of the cranial walls. Each side of the head presents two openings for eye (Lat. 

 ccidus) and ear (Lat. auris), the position of which is variable, both absoluttdy 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 ; wliile the ear is behind and a little below the eye, 

 near the articulation of the lower jaw. But the shape of the skull of Owls is sucli, that the 

 eyes are directed forward, and such birds are said t() have "eyes anterior." Owls also have 

 enormous outer ears, in some cases provided with a movable flap or ccmch, closing upon the 

 opening like the lid of a box ; and in many cases their ear-parts, and some of the cranium 

 itself, is unsymmetrical. In most birds the ear-opening is quite small, and only covered by 



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