150 LUJfBAR VERTEBRiE. 



quently make them appear hollower in the middle than the 

 others are. * 



The body of the fifth vertebrae is rather thinner than that of 

 the fourth. The spinous process of this fifth is smaller, and the 

 oblique processes face more backwards and forwards, than those 

 of any other lumbar vertebras. 



In consequence of this particular construction, the spine is 

 capable of flexion, principally in an interior and lateral direc- 

 tion, and also of extension. It ought to* be remarked, that 

 during flexion it forms a curve, and not an angle ; for, in the 

 last case, the spinal marrow would be more or less com- 

 pressed. 



The cervical vertebrae have most motion, and the dorsal the 

 least. This circumstance is fully explained by the form of the 

 different parts of these vertebrae, and the difference in the thick- 

 ness of the intervertebral substance. The necessity of fixing 

 the dorsal vertebrae is very evident : as their motion would 

 greatly interfere with the motion of the ribs in respiration. 



The lumbar vertebra3 have more motion than is commonly 

 supposed ; for, in addition to a certain degree of flexion, they 

 perform a species of rotation or twisting, which is very observa- 

 ble in persons who are diseased in one of their hip joints ; such 

 persons move their whole pelvis, by a rotation of the lumbar 

 vertebrae, to avoid moving the diseased joint. 

 — The first cause, the predisposing cause of spinal curvatures 

 is the relative feebleness of the spinal column, compared to the 

 forces exercised upon it, at the same time that the bones, by a 

 premature increase, or by a lesion of nutrition as yet little 

 known, do not acquire the degree of solidity necessary to resist 

 the action of the muscles, and especially the weight of the vis- 

 cera contained in the head and chest. There results from this 

 necessarily a curvature in some direction at- one of the points of 

 the lever. 



— The direction of the curvature will be determined by the 

 inequality of the forces brought into play around it. For with- 

 out this inequality, the curvature would be direct ; that 

 is, straight. It has place ordinarily to the left, because 



