It is true that the effect of many stimulating substances, such as alcohol, is first to excite, and after atime to weaken or depress, the actions of the heart and circulating system ; but as we know that an equal or greater degree of excite ment from exercise, from exciting passions of mind, or from inflammatory disease, may exist without producing any such subsequent de pression, we ought to regard the loss of power which follows the excessive use of such sub stances, as an ulterior effect of these substances themselves, rather than as the result of the mere circumstance of previous increased action.* Although, therefore, we consider all exertions of the irritability of muscles as necessarily im plying intervals of relaxation, and are aware of the exhaustion of irritability by excessive sti mulation, yet we do not see that the operation of those agents which augment the vital power, particularly of the involuntary muscles, is ne cessarily followed by a corresponding loss of power.
Further, it has been often alleged that the vital power of Irritability is not only expended or exhausted by excessive action, but likewise increased or accumulated by rest. But there is no evidence whatever that rest does more than merely restore the power that had been lost by previous exertion. A muscle or set of muscles which has been weakened by excessive excitement, and regained its power by rest, may remain quiescent for an indefinite time thereafter, and will not only not continue to gain power, but will gradually lose, after a time, that which it had previously possessed. The idea of the accumulation of Irritability by long-continued inaction has been thought to be supported by the fact, that the stimulating effect of Heat on all vital action, is greatest when it is applied after long-continued Cold. But this seems manifestly to be owing to the principle that the stimulating effect of heat on vital action is proportioned, not merely to the temperature that may be applied, but chiefly to the degree of change of temperature under gone in a given time; of which point many illustrations might be given, and which neces sarily implies that the effect of IIeat must be much increased by its being applied after Cold.
Another law, which may be deduced from observation of repeated exertion of living con tractile parts, is of great importance both in physiology and pathology ; viz. that the ulti mate effect of such repeated exertion, with sufficient intervals of repose, is to augment both the bulk and strength of muscular fibres, and facilitate the subsequent excitation of vital action, whether in voluntary or involuntary muscles. This is seen in the state of hyper trophy of the muscular fibres of the arms of labourers, of the legs of dancers,—of the heart, in those who have disease of the valves of the aorta,—of the bladder, in those who have disease of the prostate gland or stricture of the urethra ; and is in fact only a part of a more general law,—that the habitual exertion (within limits consistent with health) of all vital powers, is naturally attended with an increased flow of blood to the organs exerting those powers and with an increase of their nutrition. And the counterpart of this is seen in the very slow and gradual, but ultimately extreme dimi nution, not only of the vital properties, but of the bulk and characteristic appearance, of mus cular parts which have been, from any cause, kept very long in a state of absolute inaction. According to the observation of Andral, the structure of muscles may in these circum stances be so altered, that they become ulti mately hardly distinguishable from cellular texture. The act of Nutrition, and therefore the organization of muscular fibres, as well as of other living parts, is manifestly intended by nature to be, in a certain degree, dependent on the exertion of their own vital power ; and one effect of that exercise of vital power is to solicit or attract the living fluid to the part concerned in it, in a manner which the re searches of physiologists have not yet satisfac torily elucidated.
( W. P. Alison.)