426 HANDBOOK OF PHYSIOLOGY. 



Thus the body in walking is continually rising and swaying alter- 

 nately from one side to the other, as its centre of gravity has to be 

 brought alternately over one or other leg; and the curvatures of the spine 

 are altered in correspondence with the varying position of the weight 

 which it has to support. The extent to which the body is raised or 

 swayed differs much in different people. 



In walking, one foot or the other is always on the ground. The act 

 of leaping or jumping, consists in so sudden a raising of the heels by 

 the sharp and strong contraction of 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 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 imperfect idea of the 

 infinite number of combined and harmoniously arranged muscular con- 

 tractions which are necessary for even the simplest acts of locomotion. 



Action 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. 418); and the peculiar 

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

 p. 419. 



Source of Muscular Action. It was formerly supposed that each 

 act of contraction on the part of a muscle was accompanied by a correla- 

 tive 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 per- 

 formed. It has been found, however, both that the theory itself is erro- 

 neous, 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 e 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 so great as was formerly 

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

 that the waste of muscle-substance can be expressed, with unimportant ex- 

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



