14 THE EXTERIOR OF THE HORSE. 



Muscular Mechanics. These ideas can be at once applied to muscular 

 mechanics. A muscle which contracts tends to draw its two extremities equally 

 towards its centre. Each one is usually inserted upon a distinct bone. The 

 bones, however, are not all movable to the same degree. Whence it follows that 

 every muscle has a fixed and a movable insertion, the latter being situated upon the 

 bone which is displaced during the contraction. 



As the muscles of the apparatus of locomotion are voluntary, the animal 

 can at will change a movable insertion into a fixed insertion, and vice versa. In 

 other terms, a muscle, for example, which extends from the head to the arm 

 (mastoido-humeralis) can as well become a motor of the head as an extensor of 

 the arm. It suffices that the other muscles contract in order to fix and render 

 immovable the head or the arm. The multiplicity of the muscular fibres is in 

 proportion to the intensity of the contraction ; their length, on the contrary, agrees 

 with its extent. Or, if preferred, the volume of the muscles gives the measure 

 of force ; their length, that of speed. 



It is, then, we believe, an error to think that the extent of the contraction 

 of the muscles is not measured by the length of their fleshy portion, but by the 

 length of the fibres which form them. This is only partly true, for in compound 

 muscles, if the fibre be interrupted by aponeurotic or tendinous intersections, if 

 it be shorter than in simple muscles, things occur, so to speak, as if the fibre rep- 

 resented the length of the fleshy body. The extent of the contraction is the 

 result of several distinct actions which are added to one another to determine the 

 total effect. The intersections, therefore, only give greater support to the con- 

 traction of the fibre ; they furnish it with greater resistance to the tractions of 

 weight, for the muscles in which they are encountered, besides their active role 

 in locomotion, are still important passive agents in station. 



In most instances, in the members especially, the muscles are applied along 

 the length of the bones, and are found, on that account, in conditions very dis- 

 advantageous. Their manifest tendency towards parallelism with the lever-arm 

 seems to have little connection with the theoretic ideas which we have just 

 given, since a large part of their force is regarded as lost for movement. 



This is of no account, however, and it is easy to determine that this disposition 

 is, on the contrary, most fortunate, whatever may have been said concerning it. 



Indeed, if the muscle at the beginning of its contraction acts with a defective 

 incidence, this incidence becomes more and more favorable as the contraction 

 progresses, and the muscle will then profit, at the moment its action is most 

 powerful, by the entire quantity of movement already acquired by overcoming the 

 resistance. But if the insertion at the beginning be more perpendicular, the 

 muscular action, instead of acquiring intensity, will gradually lose it, and the 

 movement produced, while overcoming the resistance, will be less extended. 



On the other hand, the tendency to parallelism determines in the members 

 of animals those slender forms which we know are in accordance with speed. It 

 is certain that the muscular interstices are much more considerable as the mus- 

 cles are more removed from their bony columns and therefore more perpendicular 

 to them. This is observed in animals with massive forms and slow movements. 

 Here, the heaviness of the gait is due to the enormous size of the mass as well 

 as to the feeble degree of the contraction of the muscles. 



It must not be inferred from this that for speed we should seek absolute 

 parallelism of the muscles with their levers. It must be remembered that this 



