ii MECHANICS OF LOCOMOTOR APPARATUS 103 



long, thick muscles which appear thin. The energy these are 

 capable of developing is measured, not by the area of the section 

 vertical to their long axis (anatomical section), but by the area 

 of a section vertical to the direction of the bundles of fibres 

 (^physiological section) ; and the range of their action is measured, 

 not by their anatomical length, but by their physiological length, 

 i.e. the mean length of the muscle bundles of which they consist. 



All muscles are not inserted into the bones. The fibres of the 

 visceral organs as the heart, bladder, intestines, uterus, as well as 

 the circular fibres of the oral, pyloric, and anal sphincters are 

 only inserted into one another or into the surrounding soft parts. 



Other muscles are attached to the bone by one end, and 

 terminate at the other in soft parts, either on the skin or in the 

 mucous membrane. Such are the azygos uvulae, the levator 

 palati, the muscles of the face, the stylo -glossus, the stylo- 

 pharyngeus, etc. The muscles of the face exert a mutual traction, 

 making equilibrium with the symmetrical muscles of the other 

 side ; when the muscles on one side of the face are paralysed, the 

 mouth consequently becomes oblique. 



All the other skeletal muscles are composed of straight fibres, 

 the two ends of which are generally inserted into tendons of 

 greater or less length, by which they are attached to two distinct 

 bones of the skeleton. The majority of the muscles cross only 

 one joint, that is, they are attached by their two ends to two 

 contiguous bones, and are therefore uni- articular muscles. 

 Certain muscles, however, cross two or more articulations, and 

 are attached to more or less distant bones : these are bi- or multi- 

 articular muscles. The anterior brachial muscle is uni-articular, 

 the semi-tendinous is bi-articular, as well as the long head of the 

 biceps and certain muscles of the leg. In these cases the muscles 

 and tendons are unusually long, and as they can shorten con- 

 siderably are able to move two or more articulations simultaneously. 



When two bones are connected by a movable articulation, and 

 a muscle passes from one to the other, this forms a lever. The 

 skeleton is built up of a vast number of levers, the movements 

 of which combine among themselves, in the most various and 

 complex forms. The centre of gravity of each limb represents 

 the point of application of the resistance, that is the weight of 

 the bony lever, of the soft parts by which this is covered, and 

 of any extrinsic load which may be carried by the limb. The 

 point of insertion of a muscle or muscles upon the movable 

 segment represents the point of application of the force. Finally, 

 the fulcrum of the lever is represented by the articular surface 

 of the moving bone upon the articular surface of the fixed bone, 

 or by the ground, or any other fixed support on which the 

 limb rests. 



It is rare to find that one of two interarticulated bones is 



