602 MOTION. 



the calf -muscles, that the body is jerked off the ground. 

 At the same time the effect is much increased by first 

 bending- the thighs on the pelvis, and the legs on the 

 thighs, and then suddenly straightening out the angles 

 thus formed. The share which this action has in pro- 

 ducing the effect may be easily known by attempting to 

 leap in the upright posture, with the legs quite straight. 



Running is performed by a series of rapid low jumps 

 with each leg alternately ; so that, during each complete 

 muscular act concerned, there is a moment when both feet 

 are off the ground. 



In all these cases, however, the description of the man- 

 ner in which any given effect is produced, can give but a 

 very imperfect idea of the infinite number of combined 

 and harmoniously arranged muscular contractions which 

 are necessary for even the simplest acts of locomotion. 



Actions of the Involuntary Muscles. The involuntary 

 muscles are for the most part not attached to bones 

 arranged to act as levers, but enter into the formation of 

 such hollow parts as require a diminution of their calibre 

 by muscular action, under particular circumstances. Ex- 

 amples of this action are to be found in the intestines, 

 urinary bladder, heart, and blood-vessels, gall-bladder, 

 gland-ducts, etc. 



The difference in the manner of contraction of the striated 

 and non-striated fibres has been already referred to (p. 590); 

 and the peculiar vermicular or peristaltic action of the 

 latter fibres in some regions of the body has been described 

 at p. 345. 



Source of Muscular Action. 

 i 

 It was formerly supposed that each act of contraction 



on the part of a muscle was accompanied by a correlative 

 waste or destruction of its own substance ; and that the 

 quantity of the nitrogenous excreta, especially of urea, 

 presumably the expression of this waste, was in exact pro- 



