Although in the disposition of the parts of the osseous system of birds the plan which pervades the vertebrate type of structure is nowhere absolutely violated, yet the variations from that plan required by the peculiar exigen cies of the class are of the most striking and interesting kind. We shall successively con sider the relations of these modifications to the powers and habits of the bird as they present themselves in the vertebral axis, in the bones of the head and thorax, and in those of the anterior and posterior extremities.
The vertebral axis or spine is divisible into a cervical (fig. 1 25, a), dorsal (b), sacral (c), and caudal (d) region ; the vertebrae immediately succeeding those which bear ribs have a lateral anchylosis with the iliac bones, and therefore there is no part of the spine which possesses the characters of the lumbar vertebrae of mam inalia and reptiles.
The vertebrae are the first parts of the osseous system which make their appearance in the development of the embryo, and they are of all parts of the skeleton the most constant in their existence and general characters.
The dorsal or costal vertebrae in birds rarely form more than a fourth part of the entire vertebral column, and in some of the long necked Grallatores, as the Stork, form only an eighth part of the spine ; they have not been observed to be fewer than six nor more than eleven in number throughout the class : the latter obtains in the Swans ( Cygnus canorus et olor) and Sheldrake ; the most common numbers are seven or eight.
The dorsal vertebrae are short, as compared with the cervical : they appear broad when viewed superiorly, in consequence of the great development of the transverse processes ; but their bodies are much compressed in the lateral direction, so as to be reduced almost to the form of vertical laminae towards the sacral region. This is especially observable in the Penguins ( ilptenodytes, Catarrhactes) ; but in the Ostrich the bodies of the dorsal ver tebrae retain their breadth throughout.
The bodies are not united by intervertebral substances, but by capsular ligaments and synovial membranes ; the anterior articular cartilaginous surface is convex in the vertical direction, and concave in the transverse ; the posterior surface is the reverse. The Penguins,
however, present a remarkable exception to this rule. The posterior surface of the third dorsal vertebra is uniformly concave, to which the opposed end of the fourth vertebra presents a corresponding convexity : the ball and socket joint is continued between the several ver tebrae to the last dorsal, which is anchy losed to the sacrum. This is an interesting affinity to the Reptilia, in addition to numerous others displayed in the construction of these singular birds. In most birds the bodies of some of the middle dorsal vertebrae are an chylosed together; and in general those which are nearest the sacrum. In the Flamingo we have observed this anchylosis extending from _ _ the second to the fifth dorsal vertebra. In the Sparrow-hawk the second, third, fourth, and fifth dorsal vertebrae are conso lidated into one piece, while the sixth enjoys con siderable lateral motion both upon the fifth and seventh, which last is an chylosed to the sacrum ; so that the body can be rapidly and extensively in flected towards either side during the pursuit of prey. This structure and its uses were first pointed out by Mr. H. Earle.
The bodies of the anterior dorsal vertebrae send down processes from their inferior or ventral surfaces for the advantageous origin of the recti antici majores muscles of the neck. These processes differ from the inferior spines of the tail in not being perforated for the passage of an artery. This part of the spine is further strengthened by the extension of osseous splints from the transverse processes, which unite those of contiguous vertebrae to gether, and also by the anchylosis of the spinous processes. But where a similar ne cessity for the fixation of the trunk does not exist, as in the Struthious birds and Penguins, which cannot fly, all the dorsal vertebrae are moveable upon each other. When it is con sidered that the head, posterior extremities, and viscera are suspended in flight from this central portion of the trunk, and that it has almost exclusively to sustain the shock of the violent contractions of the principal muscles of the wings, the necessity for the mechanism consolidating the dorsal vertebra will be readily appreciated.