158 THE BONES, 



CHAPTER III. 



THE BONES IN BIRDS. 



These animals, destined for the most part to sustain themselves in the air, 

 should exhibit in the conformation of their skeleton all the conditions which 

 may favour aerial locomotion ; from this arise the differences which distinguish 

 their skeleton from that of the Mammalia — differences which will now be rapidly 

 traced. 



Veetebeal Column. Cervical vertebne. — The cervical spine represents in 

 the Bird, as in the Mammal, a kind of balancing-pole curved like an S, which 

 supports the head, and by its changes in form and direction varies the centre of 

 gravity. When a Bird rises in the air and flies rapidly, it lengthens the neck 

 and stretches out the head, to carry the centre of gravity forwards. But when 

 it rests on the ground, it makes the balancing-pole assume the natural and more 

 or less graceful inflection, by throwing the head backwards, and transferring the 

 greater portion of the weight of its body to the columns of support formed by 

 the posterior Hmbs. These displacements of the centre of gravity are executed 

 in Birds on a more extensive scale than in Mammalia ; the vertebral limbs in the 

 former are also longer, hghter, and enjoy an excessive mobility. 



The vertebrae composing it number fourteen in Fotvls, twelve in the Fir/eon, 

 fifteen in the DmJc, aijd eighteen in the Goose ; in the Swan twenty-three have 

 been counted ; — a curious variety, which singularly contrasts with the numerical 

 unity noticed as one of the most remarkable characters in Mammalia ! These 

 vertebras are generally longer than in the latter animals, and are particularly 

 distinguished by the configuration of the articular surfaces of the inferior part or 

 body. These are diarthrodial facets convex in one direction and concave in the 

 other, articulating the vertebral bodies by a veritable and reciprocal clamping. 

 In this manner, the anterior head of the body of each vertebra is replaced by a 

 facet concave on both sides, and convex vertically ; while the posterior extremity 

 of the bone bears, instead of a concavity, a facet convex in the lateral sense, and 

 concave from above to below. The inferior crest of the body (Fig. 109, 2, 2') 

 only exists in the first and last vertebrae ; but it forms a veritable spine, 

 analogous to that observed in the lumbar vertebrae of the Rabbit. The spinous 

 process (Fig. 109, 1, 1') only forms a simple crest in the middle part of the neck ; 

 it becomes more salient in the vertebras which occupy the two extremities of this 

 region. The transverse process represents on the side of the vertebra a thick, 

 obtuse, and irregular tubercle, situated under the anterior articular process, and 

 pierced at its base by a large vertebral foramen (Fig. 109, 4, 4'). It is most 

 frequently furnished with a small styloid prolongation (Fig. 109, 3, 3') directed 

 backwards and downwards, forming an epiphysis at an early period, and 

 representing a real undeveloped rib. 



The atlas has no transverse processes. This vertebra is shaped like a thin 

 ring, and is excavated on its anterior contour by a small cavity, into which is 

 received the single condyle of the occipital bone. 



The axis shows a very marked odontoid process, with a single facet under 

 that eminence. 



Dorsal vertebrce (Fig. 109, b, c). — These are seven in the Fotvl and Figeon, 



