64 



bringing to the discharge point of the propellers by 

 turning them inward. 



Finally, corresponding with action (a) of the 

 alternating side of the head-ball, the liberation of 

 these gatherings and the instantaneous formation 

 of the corresponding movements under (a). 



Remarhs. — The discharge actually alters the shape 

 of the spine, giving the final adjustment to the 

 articulations of the concaves for taking the ground. 



If the motion be of a rapid, powerful character, 

 the hinder S S will not only be first brought to the 

 point of discharge, but will first leave the ground, 

 and be the first to come down ; the whole body, 

 however close to the groimd, being in air at the 

 same moment ; but if the movement be more 

 sluggish, the front S S must the first leave and the 

 first come to the ground. 



The point that (a) {aMot even a ' ) form en air, 

 when the locomotion is once begun, is a most import- 

 ant one. 



It will often be most convenient to describe the 

 whole course of an action by referring it to the 

 head-joint movements, without going through the 

 details of the spinal workings. 



§ 69. From the position of readiness to dis- 

 charge, where (b) is carried out, three different 



