LOCOMOTOR SYSTEM 589 



of the extensor pedis and flexor metatarsi muscles, though the two 

 have a common tendon of origin. Finally, the relatively low break- 

 ing-strain of the flexor muscles is very evident. If the chief flexor 

 of the fore-limb — a muscle which perhaps does half the work in con- 

 nection with propelling the body in the gallop — breaks at a tension of 

 1,507 pounds, we may be sure from this that the breaking-strain 

 of its tendon is correspondingly low. As a matter of fact, muscular 

 rupture during life is rare, and it may be urged that such post- 

 mortem observations do not represent what occurs under the living 

 condition. That is certainly true ; nevertheless it is suggestive 

 that the low breaking-strain of the flexor muscles corresponds 

 with the frequency with which during life the flexor tendons are 

 sprained. 



Levers. — The muscles are attached to bones, and the latter 

 form various angles with each other, which are opened and closed 

 during progression. The mechanical aid introduced in order to 

 effect this is the lever. Levers are of three orders, depending upon 

 the relative position of the power, fulcrum, and weight. A lever 

 of the first order is mainly a lever of extension, the power being at 

 one end, the weight at the other, the fulcrum between. A good 

 example is the extension of the head on the neck ; the weight is the 

 head, the power lies in the neck, the fulcrum is the joint between 

 the head and neck. A lever of the second order is not common in 

 the body. The weight lies in the middle, the fulcrum and power at 

 each end. When the fore-foot is on the ground supporting weight, the 

 foot is the fulcrum, the body through the elbow- joint is the weight, 

 the power lies in the biceps muscle acting on the ulna. A lever of 

 the third order is the lever of speed and flexion. In it the power 

 lies between the fulcrum and weight. In the flexing of the arm 

 the biceps is the power, the fulcrum lies above it in the elbow-joint, 

 the weight lies below — viz., the weight of the limb. This is a lever 

 which sacrifices power for speed ; it is therefore wasteful, but necessary 

 in the limbs. 



Function of Muscles. — The muscles of the skeleton have a two- 

 fold function to perform — they move the body and they maintain 

 its attitude. Accordingly, so long as the animal is not actually 

 lying down, the muscles are in constant action, though the effort 

 required to maintain attitude is less exhausting than that 

 attached to moving and carrying weight. A standing attitude 

 is maintained by the flexible limbs being rendered rigid, and this 

 is effected by the muscles keeping the joints locked. For this 

 purpose they receive a constant stream of impulses, and if these 

 are withdrawn for ever so brief a period, the body falls under 

 its own gravity. 



It requires a greater muscular effort to maintain a standing 

 attitude in the fore than in the hind legs, owing to the fact that 

 the fore-limbs are not attached to the body by any joint, and 

 are solely dependent on masses of muscle for support. When a 

 horse sleeps in the standing attitude, his muscles not infrequently 

 lose their tonic impulses, and he falls, but always front foremost, 

 never hind foremost, and never in a heap. Long before his knees 



