. Human embryology and morphology. Embryology, Human; Morphology. 144 HUMAN EMBRYOLOGY AND MORPHOLOGY. and cervical curves also become then more marked (Fig. 114 D). The dorsal curvature and the sacro-vertebral angle are the primi- tive curves and are present in all mammals. The others are adaptations to the upright posture. The lumbar curve is most pronounced in the highly civilized races. Proportion of Cartilage and Bone.—The inter-vertebral discs form one third of the total height of the spine; the pro- portion of cartilage is greater in the lumbar than in the dorsal region and greater in t
. Human embryology and morphology. Embryology, Human; Morphology. 144 HUMAN EMBRYOLOGY AND MORPHOLOGY. and cervical curves also become then more marked (Fig. 114 D). The dorsal curvature and the sacro-vertebral angle are the primi- tive curves and are present in all mammals. The others are adaptations to the upright posture. The lumbar curve is most pronounced in the highly civilized races. Proportion of Cartilage and Bone.—The inter-vertebral discs form one third of the total height of the spine; the pro- portion of cartilage is greater in the lumbar than in the dorsal region and greater in the dorsal than in the cervical. The curvatures are due chiefly to the shape of the discs. In the lumbar region, which is convex forwards, only the lower three vertebrae are deeper in front than behind. This is true only for the higher races of mankind, for as Cunningham has shown, in lower races, as in the gorilla, only the lowest lumbar vertebra is deeper in front than behind, and thus helps to maintain the lumbar curvature. Unstable Regions of the Spine.—In about 90 °/0 of men there are 7 cervical, 12 dorsal, 5 lumbar, 5 sacral, and 4 caudal vertebrae, making 33 in all. In the remaining 10 °/0 there is some departure from the normal arrangement and these departures affect certain definite regions. I. The sacro-lumbar.—The 25th vertebra in 95 °/0 of people forms the 1st sacral; in 1% the 24th, and 3% the 26th. —24th vertebra -26th vertebra -27th. Fig, 115.—A section of the Lumbo-sacral Region of the Spine in a Foetus at the end of the 2nd month, showing the 26th vertebra forming the 1st Sacral. (After Rosenberg.) These percentages are drawn from the observations of Paterson, Eosenberg, and others who have made researches on this subject. The vertebral formula is not fixed. Eosenberg's investigations showed (Fig. 115) that it is the 26th vertebra that forms the. Please note that these images are extracted from scanned page images that may have been digitally enhanc
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