44 HAND-BOOK OF PHYSIOLOGY. 



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 attempt- 

 ing 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 imperfect idea of the in- 

 finite number of combined and harmoniously arranged muscular contrac- 

 tions 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. 36, Vol. II.); and the pecu- 

 liar vermicular or peristaltic action of the latter fibres has been described 

 at p. 36, Vol. II. 



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 expression 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 correlative 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 ^o great as was formerly supposed; 

 indeed, it is very slight. And as there is no reason to believ ethat the 

 waste of muscle-substance can be expressed, with unimportant exceptions, 

 in any other way than by an increased excretion of urea, it is evident that 

 we must look elsewhere than in destruction of muscle, for the source of 

 muscular action. For, it need scarcely be said, all force manifested in the 

 living body must be the correlative expression of force previously latent in 

 the food eaten or the tissue formed; and evidences of force expended in 



