16 THE EXTERIOR OF THE HORSE. 



According to the relative position of these three points, we recognize three classes 

 of levers. 



In the one called the first class (Fig. 7), the fixed point A occupies an inter- 

 mediate position to the two 

 others, P and R. It is 

 called inter-fixed lever. 



A certain number of 

 muscles act upon this class 

 of lever. It is exemplified, 

 for instance, in the exten- 

 sion of the head upon the 

 neck. The fixed point (oc- 

 cipito-atloid articulation) is 

 always situated, whatever 

 may be the position of the 

 FlG - 7 - head, between the resist- 



ance (centre of gravity of 



the latter) and the power (occipital insertion of the muscles of the nape of the neck). 

 The cervical ligament in our larger domestic species also equalizes the 

 weight of the head by an analogous mechanism. 



It is, however, not correct to consider the head as an inter-fixed lever at the 

 moment of flexion, as some persons think. According to their view, in fact, 

 the obstacles depend on the antagonists (extensor muscles) and the elasticity of 

 the cervical ligament. These would be the elements of resistance, the weight 

 of the head becoming, according to this opinion, an auxiliary of the power 

 (flexor muscles). 



No doubt it is sometimes thus when the head and neck are elevated, but 

 things are very different when these regions are directed towards the ground. 

 The weight of the head then becomes to the flexors a veritable resistance to be 

 overcome, and the lever is no longer of the first class ; it is of the third, as we 

 will see further on. 



Besides, in order to understand the action of a muscle well, it is necessary to 

 investigate it as if it alone were attached to the lever, without regarding the 

 manner of existence of the antagonistic ones. In most instances the latter do not 

 oppose any resistance, but allow the former to execute the movement freely. 

 The extension of the forearm, and that of the metatarsus, the femur, the pelvis, 

 the spine, etc., take place through the mechanism of an inter-fixed lever in which 

 the arm of power is, according to the case, the olecranon, the calcaneum, the tro- 

 chanter, the ischial tuberosity, the spinous or transverse apophyses of the vertebra. 

 In the economy the lever of the first class appears to be more particularly reserved 

 for extension. It is also the lever of speed, for the arm of power is never equal to 

 that of resistance : the latter is always much longer. 



In man, whose standing position is vertical, in whom falling forward is 

 easy, on account of the particular situation of the organs, the lever of the first 

 kind is, with good reason, to be considered as the lever of station. We shall see 

 that in animals it is that of the second class in which this usage can be prefer- 

 ably recognized. 



In the lever of the second class the resistance occupies the intermediate posi- 

 tion (Fig. 8). The name inter-resisting is applied to it. In this case, whatever 



