402 A MANUAL OF VETERINARY PHYSIOLOGY 



extension occurs at the beginning, and as the load is increased 

 the^extensibility becomes less. A living muscle is very ex- 

 tensible, but its elasticity, though perfect, is slight. It is there- 

 fore stretched by a slight force, and returns to its original length 

 when the extending power is removed. A contracted muscle is 

 more extensible than one which is uncontracted ; this is a pro- 

 tective mechanism intended to prevent rupture of the fibres 

 during powerful muscular effort, and is capable of ready proof 

 clinically. A ruptured muscle is incomparably less frequent 

 than a ruptured tendon. 



Tone. — In the body the muscles, as we pointed out previously, 

 are always in a condition of elastic tension — viz., they are not slack, 

 flabby masses, but slightly on the stretch, as may be demonstrated 

 by the gaping which occurs when they are divided. The use of 

 this elastic tension is to stimulate the changes which lead to a 

 contraction, also to ensure a rapid contraction without the neces- 

 sity of taking in any ' slack ' ; and, further, it is essential to the 

 proper action of the antagonistic muscles, which are thus enabled 

 to work against an elastic resistance, and so cause a smoothness 

 of motion otherwise unobtainable. The antagonistic action of 

 muscles may be well seen in a rupture of the flexor metatarsi of 

 the horse ; the unbalanced action of the gastrocnemius jerks the 

 leg behind the body, and throws the skin over the cap of the hock 

 into folds, while the Achilles tendon is kinked and bent through 

 slackness. The elastic tension of muscle is not only a valuable 

 stimulus to contraction in all varieties of muscle, but is also of the 

 greatest value in diminishing shock and strain ; nowhere is this 

 better seen than in the heart and bloodvessels. Tone is the term 

 also applied to that slightly contracted condition of skeletal 

 muscles associated with posture, which leads to the upright 

 condition of the body being maintained. It is due to the con- 

 tinuous discharge of reflex impulses from the spinal cord to the 

 muscles. In the normal posture of animals it is the extensor 

 muscles of the limb which maintain the upright attitude, and, 

 in the case of the horse, who may have to stand for weeks, the 

 extensors are severely tried. 



Work of Muscle. — If a muscle preparation is loaded with 

 successively greater weights, it is found that up to a certain 

 maximum the load absolutely increases the amount of work done 

 by the muscles. This is considered to be due to the influence of 

 the tension exercised on the fibres, the muscle adapting itself to 

 circumstances and meeting the increased weight by an increased 

 effort, as we have already seen in the case of the heart muscle. 

 By continuing to increase the weight the muscle preparation 

 becomes overloaded, and in consequence shortens less. These 

 facts are illustrated in the following table : 



