that we give to the muscles; this limit depends on the constitution; each man is by it, a more or less active leaper and runner. III. _Duration of the Contractions._ There is as it respects the duration of the contractions a remarkable difference in the muscles, according as we excite these contractions artificially or naturally. When upon a living animal or one recently killed, we excite the muscle itself, or we stimulate its nerves, the relaxation succeeds almost suddenly the contraction; neither state is ever lasting, though we continue for a long time the action of the stimulant; the effect which it has produced is immediately exhausted. When galvanism, mechanical or chemical agents are used in our experiments, the phenomenon is the same. On the contrary, when the will directs the contraction, it can sustain it for a very long time. The support of burthens, standing, &c. clearly prove this fact. When even during life, a morbid irritation is directed upon the nerves, the contraction can be very permanent, of which we have terrible proofs in tetanus. The permanence of the muscular contraction fatigues the muscle much more than alternate relaxation and contraction. Hence why when we are standing long, we contrive by turns to carry the weight of the body more upon one limb than the other. IV. _State of the Muscle in Contraction._ Muscles that contract exhibit different phenomena as follows: 1st. They evidently harden, as we may be convinced by placing the hand on the masseter, the temporal or any other superficial muscle in contraction. 2d. They increase in thickness; hence the greater prominence of all the sub-cutaneous muscles when the body is in violent action. Sculptors know this difference very well. A man at rest and a man in motion, have in their statues an exterior wholly different. 3d. The muscles when they are not confined by the aponeuroses, sometimes experience a slight displacement. 4th. They diminish in length, and thus the two points to which they are fixed approximate. 5th. Their volume remains about the same. What they lose in length, they nearly gain in thickness. Is the proportion very exact? Of what consequence to us is this insulated question, to which, since the days of Glisson, so much importance has have attached! it deserves none. 6th. The blood contained in the vessels of the muscles, especially in the veins, is in part pressed out; we increase the flow of the blood by the motions of the arm, the operation of bleeding proves both these facts.