THE INNER TISSUES OF ANIMALS. 363 



substances from which tliey arise, and therefore have greater 

 molecular mobility ; it follows that the liquid contained in 

 an active tissue has a greater average molecular mobility 

 than the liquids elsewhere; and therefore makes its way 

 through the channels of excretion faster than elsewhere : the 

 two chief products, carbonic acid and water, escaping with 

 especial facility. Hence the place becomes a place of least 

 resistance, through which the distended walls of the elastic 

 vascular system tend continually to force out an extra 

 quantity of plasma. The argument carried a 



step further, yields us an idea of the way in which not only 

 repair but also growth of the exercised tissue may be caused 

 — at least, where this tissue is one which evolves force. 

 Assuming it to be established that the force generated 

 by muscle does not result from the consumption of its nitro- 

 genous substance, but from the consumption of its contained 

 hydro-carbons and oxy-hydro-carbons ; and inferring that a 

 large amount of muscular action may be performed without 

 a corresponding loss of nitrogenous substance ; we get a 

 clue to the process of increase in a specially-exercised 

 muscle. For if osmotic exchange and osmotic distension 

 conspire to produce a more rapid passage of plasma out 

 of the capillaries into tiiis active tissue than into inactive 

 tissues ; and if, of the substances in this larger supply of 

 plasma, only the non-nitrogenous are consumed ; then there 

 must be an accumulation of the nitrogenous substances. If 

 the waste of the albuminous components of the tissue has 

 not kept pace with the consumption of its carbonaceous con- 

 tents; then there will exist in the liquid permeating it more 

 albuminous substance than is needed for its repair— there 

 will be material for its growth. The growth thus resulting, 

 however, will be limited both by the capacity of the channels 

 of supply and by the competing absorption of other active 

 tissues. So long as one muscle, or set of muscles, is 

 specially exercised, while the rest discharge but small 

 amounts of duty — so long, that is, as the quantity of 



