. Cunningham's Text-book of anatomy. Anatomy. 294 OSTEOLOGY. APPENDIX E. MORPHOLOGY OF THE LIMBS. Development and Morphology of the Appendicular Fig. 284.—Diagram to illustrate the Homologies of the Bones of the Limbs. The two limb buds of an embryo prior to flexion and rotation. The anterior or pre-axial border is coloured red; the posterior or post-axial border, blue. B. After the assumption of the foetal position. Flexion and rotation have now taken place. The red and blue lines indicate the altered position of the pre-axial and post-axial borders. C. The fully developed limbs wi
. Cunningham's Text-book of anatomy. Anatomy. 294 OSTEOLOGY. APPENDIX E. MORPHOLOGY OF THE LIMBS. Development and Morphology of the Appendicular Fig. 284.—Diagram to illustrate the Homologies of the Bones of the Limbs. The two limb buds of an embryo prior to flexion and rotation. The anterior or pre-axial border is coloured red; the posterior or post-axial border, blue. B. After the assumption of the foetal position. Flexion and rotation have now taken place. The red and blue lines indicate the altered position of the pre-axial and post-axial borders. C. The fully developed limbs with the flexor aspects directed towards the reader. The coloured lines indicate the effect of the torsion of the upper segment of the limb through quarters of a circle. The paired limbs first appear in the human embryo about the third week as small buds on either side of the cephalic and caudal ends of the trunk. That these outgrowths are derived from a large number of trunk segments is assumed on the ground that they are supplied by a corresponding number of segmental nerves, and the circum- stance that they are more particularly associated with the ventral offsets of these nerves would point to the conclusion that they belong rather to the ventral than the dorsal surface of the body. At first the surfaces of these limb buds are so disposed as to be directed ventrally and dorsally, the ventral aspect correspond- ing to the future flexor surface of the limb, the dorsal to the extensor side. At the same time, the borders are directed head- wards (pre-axial), and tailwards (post-axial). As the limbs grow, they soon display evidence of division into segments corre- sponding to the hand and foot, forearm and leg, upper arm and thigh. Coincident with this (about the second month) the cartil- aginous framework of the limb is being differentiated. The disposition of these cartilages furnishes a clue to their homo- logies. In the fore-limb the radius and thumb lie along the pre-axial
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