ANATOMY OF A BIRD. 



241 



d' 



these air-spaces and those which are derived from the lungs enables the contained air to undergo the 

 necessary exchanges with the surrounding medium. 



It was long ago observed by the famous German anatomist, Johannes Miiller, that " it has often 

 been a subject for complaint that the anatomical characters of birds are so constant that they are of but 

 slight assistance in the labours of the 

 zoologist." The truth of this will very 

 forcibly strike any one who comes to the 

 study of the skulls of birds, after having 

 examined a series of skulls in mammals, 

 so that the seemingly trivial variations 

 to which anatomists have directed at- 

 tention are in truth those which are, in 

 birds, often of the most importance. 



The skull, then, is, as compared 

 with the rest of the body, small ; but 

 that portion which contains the brain is 

 relatively larger to the face than it is in 

 any living mammal. The orbits, or 

 cavities in which lie the eyes, are very 

 deep, in consequence of the small ex- 

 tent to which the walls of the brain- 

 case extend forwards. The cavities of 

 each side are separated by a partition 

 (inter-orbital septum), which is more or 

 less bony ; the nasal bones are short, so 

 that the nasal orifices (anterior iwres) 

 are placed near to where the beak joins 

 the face. Of the four bones which 

 bound the great opening at the back 

 of the skull for the passage of the 

 spinal cord, three take part in the 

 formation of the single ball-like pro- 

 jection, or condyle, by which the skull 

 is hinged on the vertebral column. In 

 this point, the skull of birds offers a 

 striking point of dissimilarity to that 

 of mammals, in which there are two 

 condyles, one on each side of the great 

 opening (of). Another point in which 

 birds do not resemble mammals is in the 

 mode by which the lower jaw is hinged 

 on the skull. This is in the case of birds 



. SKELETON OF EAGLE. (Reduced. After Milne-Edwards.) 



effected by a bone, which, being more 



i ] -i j ,1 



Or leSS Square 111 Sliape, naS gamed the 



.. 

 nailie Ol the OUddrate (O). In mammals 



the skull proper and the lower jaw are 

 directly connected. This quadrate bone is connected by a long narrow bar (quadrato-jugal) with the 

 bones which go to form the " beak," and also, by a narrow bone directed inwards, with the bones which 

 lie in the middle line of the base of the skull, and form the hard palate. The connections between 

 these bones are often of such a kind as to allow of the upper jaw, or upper half of the beak, being 

 movable on the rest of the skull, the upper bones of which are so completely united together as to 

 form a very firm point of support. In the Parrots this arrangement is carried to an extreme, for 

 the slender bones (nasals and processes of the pre-maxillaries) which connect the upper jaw with the 



) p e lvis: (O Coracoid; (drl Dorsal ribs; (so Sternal ribs: (P> Uncinate processes; 

 (co) Coccyx ; (r) Radius; (u> Ulna: (d) first phalanx of chief digit of the wing: (d') secord 

 phnlanx of chief digit of the wing; (d"> Phalanges of lower digit; W") Pollex: (ca) 

 Carpus; (/) Femur; <fu] Furcula; (A) Hunierug; ippi Postorbital process; ) Tarso- 

 metatarsus; (t> Metatarsus; (ma) Lower jaw; irn--) Metacarpus; (s) Scapula; (pal Pha- 

 langes of foot; (fi) Fibula; (pt) Patella; (st) Sternum ; (tij Tibia- 



