SOURCE OF MUSCULAR ACTION. 473 



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 producing 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 manner 

 in which any given effect is produced, can give but a very im- 

 perfect idea of the infinite number of combined and harmoni- 

 ously arranged muscular contractions which are necessary for 

 even the simplest acts of locomotion. 



Actions of the Involuntary Muscles. The involuntary mus- 

 cles 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. Examples of this action are 

 to be found in the intestines, urinary bladder, heart and blood- 

 vessels, gall-bladder, gland-ducts, &c. 



The difference in the manner of contraction of the striated 

 and non-striated fibres has been already referred to (p. 463) ; 

 and the peculiar vermicular or peristaltic action of the latter 

 fibres in some regions of the body has been described at p. 276. 



Source of Muscular Action. 



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 ex- 

 pression of this waste, was in exact proportion to the amount 

 of muscular work performed. It has been found, however, 

 both that the theory itself is erroneous, and that the supposed 

 facts on which it was founded do not exist. 



It is true that in the action of muscles, as of all other parts, 

 there is a certain destruction of tissue or, in other words, a 

 certain "wear and tear," which may be represented by a slight 

 increase in the quantity of urea excreted : but it is not the 

 cor relative expression or only source of the power manifested. 

 The increase in the amount of urea which is excreted after 

 muscular exertion is by no means so great as was formerly 

 supposed ; indeed, it is very slight. And as there is no reason 



