When one or several groups of muscles contract themselves strongly in order to perform a function requiring force, or to overcome an obstacle, to draw or push away a body, the name effort has been given to this action of the muscles. There is effort in walking, climbing, running, and a great number of other functions. Whatever muscles take part in effort they must have a point of support or fulcrum, directly or indirectly upon the skeleton of the trunk, that is on the spinal column and the bones of the thorax. An effort is always preceded by an inspiration which dilates the thorax, thus rendering the bones of which it is composed immovable by the contraction of the inspiratory muscles, and furnishing a fixed point to the muscles attached to these bones. Thus, one after another, the greater part of the muscles of the system joins in a movement of which sometimes the arm or the hand only is the immediate instrument The proof of this is the impossibility of making a movement distinct from that which is the immediate object of the effort, without this effort ceasing or diminishing. When it is at its highest point, respiration is suspended, the glottis closes or remains slightly open, according to the nature and the degree of intensity in the effort; the inspired air distends the lungs, and if a part escapes it is too small to lessen the expansion of the chest; the abdominal viscera are compressed from above by the diaphragm in front, and laterally by the muscles of the abdomen. During certain efforts the air is expelled slowly through the glottis, and when the movement terminates suddenly with redoubled force, the expiration is accomplished rapidly and sometimes in the form of a cry. The sailor who hauls in a rope, or the baker as he with difficulty raises the dough to throw it back into the kneading-trough, accompanies the movements which he executes with a cry of which the rhythm expresses the different periods of the effort.