PHYSIOLOGY OF MOVEMENT. 731 



length of the muscle-fibre corresponds to the degree of movement which 

 the muscle has to produce. It does not necessarily follow that a muscle 

 whose points of origin and insertion are wideiy separated should be a 

 long muscle, since the interval between these two points may be largely 

 taken up by tendons. 



3. Animal Locomotion. — The essential factor for animal locomotion 

 consists in the movement of the centre of gravity of the body. By the 

 term " the centre of gravity " is understood the point about which all 

 the matter composing the body may be balanced. The attraction of 

 gravity tends to draw every particle of matter downward in a vertical 

 line. The factors of this force may be, therefore, regarded as the sum of 

 an almost infinite number of parallel forces, each of which is acting upon 

 one of the molecules of which that body is composed. Just as the 

 resultant of the force exerted by tw.o horses harnessed to a swingle-tree 

 is equal to the sum of the forces exerted by the horses but applied at a 

 single point at or near the centre of the swingle-tree, so, also, the sum 

 of the forces of gravity ma}' be regarded as acting upon a single point 

 which is near the centre of gravity of that body. In other words, the 

 weight of the body may be considered as concentrated at the centre of 

 gravity. When the centre of gravity is supported the whole body will 

 be in a state of equilibrium ; or when the line of direction of the force 

 of gravity, which is thus a vertical line passing through the centre of 

 gravity, falls within the base of the body, or base on which the body 

 stands, it is then said to be stable. 



In all regular bodies the centre of gravity will coincide with the 

 central point, while in irregular bodies it will be nearest to that part in 

 which the greatest weight is concentrated. As the centre of gravity of 

 the animal body is within the body it can be directly supported. The 

 stability of the body will be greater the broader the base and the nearer 

 the centre of gravity to the support. The animal body when standing is, 

 however, only at best in a state of unstable equilibrium ; for when slightly 

 displaced from its position of equilibrium, it tends to fall still farther 

 from that position, owing to the fact that the disturbance has lowered 

 the centre of gravity, and equilibrium is not restored until it reaches its 

 lowest possible point. An animal in a recumbent position is in a state 

 of neutral equilibrium ; when its position is changed it tends neither to 

 return to its former position nor to fall farther from it. Stable equi- 

 librium is when a body is so supported that when slightly displaced from 

 its position of equilibrium it tends to return to that position. Such a 

 condition can only occur when displacement raises the centre of gravity. 

 The pendulum is an example of stable equilibrium. 



Standing is thus a condition of unstable equilibrium in which the 

 centre of gravity is supported from the fact that the line of direction 



