i c 2r Dr. Wollaston on the salutary Effects 
fatiguing efforts have consequently been substituted for motions 
that are agreeable, and even directly invigorating, when duly 
adapted to the strength of the invalid, and the peculiar nature 
of his indisposition. 
The explanation which I am about to offer of the effects of 
external motion 'upon the circulation of the blood, is founded 
upon a part of the structure observable in the venous system, 
the mechanical tendency of which cannot be doubted. The 
valves which are every where dispersed through those ves- 
sels, allow free passage to the blood, when propelled forward, 
by any motion that assists its progress ; but they oppose an 
immediate obstacle to such as have a contrary tendency. 
The circulation is consequently helped forward by every 
degree of gentle agitation. The heart is supported in any la- 
borious effort that may have become necessary, by some 
obstacle to its exertions ; it is assisted in the great work of 
restoring a system, which has recently struggled with some* 
violent attack ; or it is allowed, as it were, to rest from a la- 
bour, to which it is unequal, when the powers of life are 
nearly exhausted by any lingering disorder. 
In the relief thus afforded to an organ so essential to life, 
ail other vital functions must necessarily participate ; and the 
various offices of secretion, and assimilation, by whatever 
means they are performed, will not fail to be promoted during 
such comparative repose from laborious exertion. 
Even the powers of the mind itself, though apparently, 
least likely to be influenced, by mere mechanical means, are 
manifestly, and in many persons, most immediately affected* 
by these kinds of motion. 
It is not only in cases of absolute deficiency of power to 
