94 TEXT-BOOK OF PHYSIOLOGY. 



tion of the muscle in consequence of activity, as they disappear on 

 the restoration of the muscle to its normal condition. 



Work Done Daily. The muscle system in its entirety is to be 

 regarded as a machine for the transformation of potential into kinetic 

 energy, and in so doing accomplishes work. Through the inter- 

 mediations of the bones of the skeleton which play the part of levers 

 the individual not only changes his position in space, but overcomes 

 to some extent the resistances offered by the environment. The 

 employment of artificial levers, tools, as distinguished from natural 

 levers, bones, materially adds to the effectiveness of the muscle 

 machine. The amount of work which a man of average physical 

 development weighing 72 kilos can perform in eight hours has been 

 variously estimated. It will naturally vary according to the character 

 of the occupation. If the work done be calculated from the number 

 of kilograms raised one meter, the average laboring-man performs 

 about 300,000 kilogrammeters. 



SPECIAL ACTION OF MUSCLE GROUPS. 



The individual muscles of the axial and appendicular portions of 

 the body are named with reference to their shape, action, structure, 

 etc.; e. g., deltoid, flexor, penniform, etc. In different localities a 

 group of muscles having a common function is named in accordance 

 with the kind of motion it produces or to which it gives rise: e. g., 

 groups of muscles which alternately diminish or increase the angular 

 distance between two bones are known respectively as flexors and 

 extensors; such muscle groups are usually found in association 

 with ginglymus joints. Muscles which rotate the bone to which 

 they are attached around its own axis without producing any great 

 change of position are known as rotators, and are found in association 

 with enarthrodial or ball-and-socket joints. Muscles which impart 

 an angular movement to the extremities to and from the median line 

 of the body are termed adductors and abductors respectively. 



In addition to the actions of individual groups of muscles in pro- 

 ducing special movements, in some regions of the body, several 

 groups of muscles are coordinated for the accomplishment of certain 

 definite functions; e. g., the functions of respiration, mastication, etc. 

 The coordination of axial and appendicular muscles enables the 

 individual to assume certain postures, such as standing, sitting, and 

 lying; to engage in various acts of locomotion, as walking, running, 

 dancing, swimming. 



Levers. The function or special mode of action of individual 

 muscles can be understood only when the bones with which they are 

 connected are regarded as levers whose fulcra or fixed points lie in 

 the joints where the movement takes place, and the muscles as sources 



