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 °/ of men 

 there are 7 cervical, 1 2 dorsal, 5 lumbar, 5 sacral, and 4 caudal 

 vertebrae, making 33 in all. In the remaining 10 °/ there is 

 some departure from the normal arrangement and these departures 

 affect certain definite regions. 



I. The sacro-lumbar. — The 25th vertebra in 95 °/ 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 



