. Natural history object lessons : a manual for teachers. e-ing solid, or filled withmarrow, are hollow; andfor these reasons, as sup-ports for the powerfulmuscles they must bestrong, and for the at-tachment of these mus-cles they must be large;while at the same timethey must not be tooheavy, or flight wouldbe impossible. In thesecond place the bonesof the fore limbs aremodified to suit the re-quirements of a winginstead of a hand or bones of the fore-arm are lengthened, theincrease in length bearing relation to the power of flight,while the hand is reduced to a single piece for the su
. Natural history object lessons : a manual for teachers. e-ing solid, or filled withmarrow, are hollow; andfor these reasons, as sup-ports for the powerfulmuscles they must bestrong, and for the at-tachment of these mus-cles they must be large;while at the same timethey must not be tooheavy, or flight wouldbe impossible. In thesecond place the bonesof the fore limbs aremodified to suit the re-quirements of a winginstead of a hand or bones of the fore-arm are lengthened, theincrease in length bearing relation to the power of flight,while the hand is reduced to a single piece for the supportof the large feathers of the wing. A rudimentary thumband the vestiges of a third finger are, however, present. Inthe hind limbs the thigh-bone is short and directed forwards,the leg-bone is longer and composed of but one strong bone,while the ankle and foot-bones are represented by one bone,which practically forms a lower leg-bone. It is the length * In the sheep and ox two are mere rudiments, and in the camel andgiraffe two toes only are Kg. 101.—Skeleton of Vulture. 150 NATURAL HISTORY OBJECT LESSONS. of this bone which determines the height of the bird whenstanding. In the tortoises and turtles the skeleton undergoes a pecu-liar modification, to assist in the formation of a great plate,called the carapace, which covers the animal as with an um-brella. This carapace is formed of the vertebral column,
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