PHYSIOLOGY OF MOVEMENT. 723 



arranged in several layers, an oblique, circular, and longitudinal layer 

 being in nearly all cases distinguishable ; all these sets of fibres acting 

 simultaneously, the result is to diminish the capacity of the cavity which 

 they inclose. They thus aid in various motions of animal life, such as 

 the propulsion of the blood from the heart and in assisting its onward 

 passage through the arteries, the evacuation of the bladder and rectum, 

 the emptying of the pregnant uterus, and various other operations which 

 have been already alluded to. In addition to this group of muscles, the 

 sphincters likewise have no definite origin or insertion, but are found at 

 the various openings of the body, whether the anus, urethra, or mouth, 

 and several other localities. The muscular fibres of the sphincters are 

 circular, and by their contraction serve to close the orifices of these 

 several openings. 



Of the muscles with definite origin and insertion, either the origin is 

 fixed or both origin and insertion may be movable. In many cases be- 

 longing to the former of these groups the origin is only fixed during 

 muscular action ; thus, in the case of the palato-pharyngeal muscles their 

 characteristic action is only rendered possible by the fixation of their 

 origin through the contraction of the levator palati muscle. 



Again, both origin and insertion may be movable, the part moved 

 being usually under the control of the will. Thus, in the case of the 

 sterno-mastoid muscle, through its contraction the head may be depressed 

 or the chest elevated. 



Movement of the animal parts or of the entire animal body is ren- 

 dered possible through the manner in which the skeletal muscles are 

 inserted in the long bones by which lever motion is possible, the bones 

 being regarded as levers, the joint as the fulcrum, the insertion of the 

 muscle the point of application of the power, and the centre of gravity 

 of the bone with the resistances overcome by its motion as the load. 



Thus, muscles arising in one bone and inserted in another will, in 

 their contraction, either move both bones toward each other, or, if one be 

 fixed, will approach the movable to the fixed bone. From the definition 

 of the power-arm of a lever, it is evident that in general the direction of 

 muscles relative to the levers on which they act is very disadvantageous, 

 since their course is almost always more or less parallel to the bony levers. 

 This parallelism is diminished by the swelling of the articular extremi- 

 ties and by the development of more or less marked eminences, such as 

 the olecranon or the trocanters, or by the presence of sesamoid bones. 

 In movements of flexion, however, this parallelism becomes diminished 

 according to the degree of flexion, so that, therefore, at the termination 

 of the act of flexion the muscles are more favorably situated for the de- 

 velopment of power. Certain muscles, however, such as the muscles of 

 mastication, the flexors of the head, the psoas muscles, and the abductors 



