726 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



joint above, far from the insertion of the muscle. Hence, the power-arm is 

 greatly increased. Thus, in the horse, while the foot is on the ground in the con- 

 traction of the extensors the point of application of the weight is in the hock- 

 joint, the point of application of the power in the caleaneum, where the extensors 

 are inserted, and the pastern -joint the fulcrum ; hence the power-arm is long, and, 

 although motion is slow, it is accompanied by a corresponding increase in power. 

 If, however, the hind foot is not on the ground, but is extended as in kicking, then 

 the fulcrum is in the hock-joint, and the power-arm is now short and power is 

 exchanged for velocity. 



The first class of levers may be represented in such movements as in nodding 

 the head, where the fulcrum is the articular surface of the atlas, the weight being 

 found in the back of the head when the throat muscles contract, in the front of 

 the head when the posterior neck muscles contract (Fig. 290). Movement due to 

 the action of levers of the second class is seen when the body is raised up on tiptoe 

 by the muscles of the calf (Fig. 291). 



All three orders of levers may come into play in the action of the human 

 elbow-joint. Thus, the first class is illustrated when the forearm is extended on 

 the arm through the contraction of the triceps muscles ; in this instance, the hand 

 is the weight, the elbow -joint the fulcrum, and the insertion of the triceps in the 

 olecranon the power (see upper diagram Fig. 287). 



If the hand rest on the table and the body be raised on it, then the hand 

 is the fulcrum, the triceps is the power, and the humerus, at its articulation in 

 the elbow-joint, the weight, thus illustrating the action of a lever of the second 

 class (see middle diagram 287). The third order, as already mentioned, comes 

 into play when the forearm is flexed on the arm. 



In the horse the extensors of the forearm (A Z>, B D, and G D, Fig. 

 292) act as levers of the first class, the power-arm being the distance 

 between the summit of the olecranon and the centre of the humero- 

 radial articulation, which forms the fulcrum, while the weight-arm is 

 represented by the length of the radius. In man the triceps brachialis 

 (B, Fig. 293), which is the analogue of the olecranon muscles of quad- 

 rupeds, acts also as a lever of the first class, the power-arm, however, 

 being much shorter in man. In the posterior extremity of the horse 

 (Fig. 294), the glutens medius, the fascia lata, the triceps cruralis, the 

 Mfemero-calcaneus, the vastus externus, etc., are also examples of the 

 first class of levers. In the case of the gluteus medius (A B, Fig. 294) 

 the power-arm is the distance from the trochanter to the centre of the 

 acetabular articulation, which is the fulcrum, while the weight-arm is the 

 length of the femur. For the gastrocnemius the power-arm is the 

 distance from the summit of the caleaneum to the centre of the hock- 

 joint, which is the fulcrum, while the weight-arm is the length of the 

 metatarsus. The first class of levers is thus mainly represented by the 

 extensors. 



Levers of the third class are mainly represented by the flexors. In 

 the anterior extremity of the horse (Fig. 295) the infraspinatus, the 

 biceps flexor, the metacarpal flexor, and the flexor pedis are all examples 

 of muscles whose action operates through levers of the third class, in 

 each instance the power acting between the fulcrum and the weight. In 

 operations of levers of the third class power is exchanged for velocity 

 of motion, from the fact that tlie power-arm is alwa}~s shorter than the 



