XIV 



preponderance at the rear, thrust the body forward over the 

 bearers, which raise it at the front of the curvature. 



All the tractions gather in diagonal action, but discharge 

 in collateral action. This collateral action is only moment- 

 ary in the movements produced by alternate diagonal 

 counteractions, but if the two possible sets of counterac- 

 tions are, suppose, to work simultaneously, they reciprocally 

 suppress the diagonal elements of shape, and the alternate 

 helical forms are replaced by undulations in the perpen- 

 dicular plane. 



In these undulations the force of the diagonal elements 

 is yet present, but the acting tractions are made collateral 

 even before the discharge. 



It is this undulating shape which the spine of the higher 

 animals assumes, more or less, for all springs from two hind 

 feet at once, and its perfect production is required for a per- 

 fect halt from any kind of movement. 



The position of such a halt is that in which the body is 

 perfectly gathered for making, on the instant, any move- 

 ment of which it is capable, and is what is meant by " set- 

 ting-up " when applied to an attitude of immobility. 



This position may be produced either by forcing one 

 of the sets of counteractions beyond its limits without 

 discharge, in which case the other set accepts the excess of 

 gathering, and both gatherings thus become formed ; or by 

 commencing with the formation of both sets at the same 

 time. 



The movements of the snake are caused by a succession 

 of springs from an indefinite number of what may be called 

 torsion curves, of two curvatures each ; but in man and the 

 higher animals the spinal curves are limited to three : — one 

 in the back, which answers to the chest curve in the snake ; 

 one in the loins, which is not complete until by moving the 

 hinder limbs it has compounded itself with the chest 

 curve ; and one in the neck, which combines itself with the 



