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TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



longed distally as slender tendons; in this way the 

 center of gravity of the limb is comparatively high and 

 we get the rapid oscillation of a pendulum in which the 

 bob or weight is placed near the pivot. 



In graviportal animals, on the contrary, the limb 

 muscles are more evenly distributed down the limb 

 and the slender tendinous portions are shorter, so that 

 we have the slower oscillation of a pendulum in 

 which the weight is moved down the shaft. But the 

 free swing of the limbs is alternately accelerated and 



FiGUBE 663.- 



.'-'C 



-Angles of muscular insertion and "parallelogram of forces. 

 After Gregory, 1912.1 



inversely proportioned to the amount of inert con- 

 nective tissue that is interspersed with the striped 

 muscle fibers. Muscles when stretched serve as 

 ligaments and help to prevent dislocation of the joints, 

 as well as to transmit the pull of other muscles. Thus 

 the biceps muscle and tendon assist in preventing 

 dislocation of the shoulder; the quadriceps femoris 

 and its tendons prevent dislocation of the knee joint; 

 the serratus and its allies help to sling the body between 

 the shoulders. 



In general, long and slender muscles, such 

 as the sartorius, exert a small force over a 

 long range, while short and thick muscles, such 

 as the gluteal mass in many ungulates, exert 

 a great force through a short range. When 

 the muscle fibers are set obliquely to the 

 long axis of the muscle, as in the gastrocnemius, 

 the muscle contracts slowly but with increased 

 power. The contractile force is highest when 

 a muscle is stretched to its full "physio- 

 logical length" (that is, the greatest length 

 it ever assumes during life), and the greatest 

 force and velocity of contraction are developed 

 when the movement of the muscle is checked 

 during the initial stages and when the resist- 

 ance is suddenly diminished. These prin- 

 ciples operate constantly in the limbs, espe- 

 cially in the straightening of the knee joint, 

 and in many cases where there is a sudden 

 snap or jerk, as in the action of the flexors 

 of the digits in a running horse. 



It is a familiar dictum of elementary 

 mechanics that in considering the action 

 of levers "what is gained in speed is lost 

 in power, and what is gained in power is 

 lost in speed." In the skeleton these recip- 



If the muscles contract at equal rates and the angle of insertion is acute (ABD) the extremity of a 

 bone is moved through a wide arc (CC), with feeble propulsive power {BE) and great lifting 



power (.-li?). As the angle of insertion increases (a6d) the extremity of the bone is moved through j^QCal relations of " POWCr " and "sDCed" 

 a smaller arc (cc') with greater propulsive power (iir) and reduced lifting power (a6). If the muscles rrxj i. lu+1 1+' J" 



contract at unequal rates a slowly contracting muscle (.BD) inserted at a small angle may move ^-^e aiiected not Only by the relative QIS- 



the insertion point slowly over a wide arc (B£'). whereas a more rapidly contracting muscle (6(i) tancCS between the fulcruni (joint) the forcC 



may move the insertion point more quickly but through a smaller arc (bb') . zci i\ i.iii '■, 



(of the muscle), and the load or resistance, 



retarded by the actions of the muscles, and the fore- 

 and-aft movement is complicated by tw-isting, ab- 

 duction, and adduction of the various segments of the 

 limb. 



When isolated for laboratory experiments, muscles 

 react if stimulated according to relatively simple 

 mechanical and physiological laws. An isolated mus- 

 cle will contract upon stimulation to a varying fraction 

 of its own length, from one-fifth to one-third. The 

 force of a muscle — that is, its ability to overcome 

 inertia at a given instant — is proportional to the num- 

 ber of muscle fibers cut in a cross section of the 

 muscle; but the work that a muscle can perform 

 during the entire period of its contraction is propor- 

 tional both to the area of the cross section and to the 

 length of the muscle — that is, the total work performed 

 is proportional to the mass of the muscle; it is also 



but also by the "angle of insertion" of the muscle. 

 If a man pulls a heavy door directly toward him 

 (at right angles to the door) he will naturally find 

 that it wUl move much more easily than when he 

 puUs it at an oblique angle; but in the second case, 

 although the effort required is greater, the movement 

 of the door is much faster. The "angle of insertion" 

 of a muscle is formed by the long axis of the muscle, 

 its point of insertion, and the line between the inser- 

 tion point and the fulcrum or joint. (See fig. 664.) 

 If the angle of insertion is large, as in the gastrocnemius 

 of graviportal animals, great power is secured. If 

 tlie angle of insertion is small, as in the flexors and 

 extensors of the digits, the movement of the insertion 

 point may be very rapid. 



From the diagram above referred to we learn that 

 if two muscles contract at the same rate the lesser the 



