This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(2027). The class of Vertebrate animals which now offers itself to our notice contrasts remarkably with the cold-blooded and apathetic inhabitants of the water, and even with the slow-moving Reptile, that languidly crawls upon the surface of the ground, or drags on an amphibious existence in the marsh or on the shore. The Bird, ordained to soar into the regions of the air, and not only to sustain itself in that thin medium, but to skim from place to place with astonishing rapidity, needs a strength of muscle and activity of limb even greater than that conferred upon the mammiferous quadruped. Senses of the utmost acuteness are now requisite, combined with instinct and intelligence of a high order; and accordingly, both as regards their faculties and enjoyments, the feathered tribes far surpass the other oviparous Vertebrata.
(2028). Next to that improvement in the condition of the nervous system, which we have all along been able to trace advancing pari passu with the increase of sagacity and the expansion of the bodily faculties, the most remarkable circumstance observable in the economy of Birds is the elevated temperature of their bodies and the heat of their circulating fluids. In the Reptile an impure and semioxygenized blood was slowly propelled through the system from the undivided ventricle of their trilocular heart, and we found their energies, their instincts, and their affections proportionately feeble and obtuse; but now, not only does the heart become divided into four cavities (one ventricle being appropriated to transmit venous blood to the lungs, while the other drives a pure and highly-arterialized fluid in copious gushes to the remotest regions of the body), but, as though even this was not sufficient to meet the necessities of the case, the whole interior of the Bird is permeated by the atmospheric air, which penetrates even into the bones; and the respiratory function being thus rendered as complete as possible, all parts of the muscular system are abundantly supplied with blood arterialized to the utmost, and every fibre, quivering with life intense, is ready to exert that vigorous activity which brings down the falcon upon his quarry like a thunderbolt from the clouds, or sustains the migratory bird through long and perilous journeyings.
(2029). But increase of muscular energy is by no means the only consequence resulting from more perfect respiration and a consequently increased temperature of the blood: the clothing of the body must now be changed for a warmer covering than scales or horny plates; feathers are therefore at once provided, as the lightest, warmest blanket that could be given: maternal care, which to the cold-blooded Ovipara would have been a useless boon, can now be beneficially exercised; the eggs, no longer left to chance, are cherished by the vital heat of the parent; and the callow brood, during the first period of their lives, are dependent for support upon the watchful attentions of the beings from whom they derived their existence.
(2030). The skeleton of a vertebrate animal formed for flight must obviously be constructed upon mechanical principles widely different from any that have yet come under our notice. The utmost lightness is indispensable; but still, in a framework which has to sustain the action of muscles so vigorous, strength and firmness are equally essential: it is in combining these two opposite qualities that the human mechanician displays the highest efforts of ingenuity, and by the scientific disposition of his materials exhibits the extent of his resources and the accuracy of his knowledge; but let the best-informed and most ingenious mechanic carefully and rigidly investigate the skeleton of a bird, and we doubt not that in it he will find all his art surpassed, and derive not a little instruction from the survey.
(2031). In the spinal column of a bird we find three principal regions, each of which will merit distinct notice.
(2032). The anterior or cervical region is exceedingly variable in its proportionate length, and forms the only flexible portion of the spine: it performs, indeed, the office of an arm, at the extremity of which the beak, the chief instrument of prehension, is situated. The number of vertebrae entering into the composition of this part of the spinal column is very variable: in the Swan there are as many as twenty-three; in the Crane, nineteen; while in the little Sparrow nine only are met with: their bodies are joined together by articulating facets enclosed in synovial capsules, and not by the interposition of intervertebral substance; an interarticular cartilage, however, is generally met with, by which the movements of the chain are facilitated. The spinous and transverse processes are short; while the oblique processes, united by articulating surfaces, limit the mobility of the neck.
(2033). Although this portion of the spine is very properly designated the "cervical region," we are not on that account to imagine that the vertebras composing it are unprovided with ribs: on the contrary, rudimentary costal appendages are generally found connected with their transverse processes, which, in the young bird, are obviously separate elements, although they afterwards become united by anchylosis.
(2034). But if flexibility is thus abundantly provided for in the cervical portion of the vertebral column, it is quite evident that in the thoracic portion of the skeleton, which has to support the framework of the wings, and sustain the efforts of the muscles connected with flight, firmness and rigidity become essential requisites; and accordingly everything has been done to prevent those movements which in the neck were so advantageously permitted. The bodies and spinous processes of the contiguous vertebras are therefore here firmly consolidated together by anchylosis; and, moreover, splints of bone, derived from the transverse processes, overlap each other and still further add to the stability and strength of the back.