'64 INTRODUCTION. 



gradually assuming a horizontal direction in Man, or a vertical direction in 

 quadrupedal Mammalia ; so that, instead of being confined to one point, 

 the centre of motion is diffused over the space of the three or four last 

 dorsal, and two or three first lumbar vertebrae, an abruptly acute curve 

 being thus avoided. 



Professor Owen observes, that " the relation which the structure of 

 the vertebral column bears to the mode of progression of a quadruped 

 is extremely interesting, and enables us to judge, in some degree, from 

 the spine alone, of the locomotive faculties of a fossil species." If we 

 attend to the progressive motion of any heavy animal, as the Ox, we shall 

 find the flexibility of the vertebral column (at least of its dorsal and 

 lumbar portions) to be very restricted, and its centre of motion indefi- 

 nite : it seems destitute of that suppleness which we see so marked in 

 the Weasel, or the Cat. Now, if the dorsal and lumbar vertebrae be 

 examined, they will be found short, and with only a thin layer of 

 elastic cartilage intervening between their bodies ; while their large, 

 strong, spinous processes have no point between them, to which they 

 definitely converge. In animals endowed with great flexibility of body, 

 as the Cat, the Leopard, &c., this converging point is clearly marked, 

 and the oblique bearing, in a direction opposing that of the dorsal and lum- 

 bar vertebrae, is very decided : added to which, the bodies of the ver- 

 tebrae are, comparatively, longer, and the layer of cartilage, interposing 

 between each, is, relatively, of greater thickness than in the Ox. 

 Some animals have no centre of motion in the back, as the Arma- 

 dillo, the Chlamyphorus, &c. ; and in these the spinous processes are all 

 equally directed backward. The progressive motion of such animals is 

 automaton-like : their legs seem to go by means of machinery, the 

 action of which affects no other part of the body. No inflexions of the 

 spine accompany the movements of the limbs : the two extremities of 

 the vertebral column are not alternately raised and lowered as in the 

 bounding Leopard ; but the back preserves its uniform level, however 

 rapid may be the motion of the limbs. It is from this circumstance that 

 the rapid movements of the Armadilloes, in the gardens of the Zoological 

 Society of London, have never failed to excite surprise. 



The next part of the spine, for examination, is the sacrum. 



The narrowness of the sacrum, and the deficiency, in expansion and 

 volume, of the iliac bones, and of the pelvis altogether, in the two most 

 anthropomorphous of the Simiae, compared with similar parts in the human 

 subject, are worthy of especial notice. (See figs. 64, 65, 66, 67.) 



The os sacrum (a), or that solid portion of the vertebral column which 

 is regarded as its fourth division, is, in the human species, somewhat 

 triangular in shape ; concave on its inner aspect, and convex at its back : 

 it originally consisted of five distinct vertebrae : at an early period, how- 



