298 PHYSIOLOGICAL DEVELOPMENT. 



extra nutriment to the structures adjacent, lead, other things 

 equal, to thickening or induration. The data for the con- 

 clusion are these : — Through the walls of the capillaries the 

 liquid plasma of the blood continually oozes. The oozing 

 is partly osmotic and partly mechanical — partly due, that is, 

 to the exchange of the unlike liquids that lie inside and out- 

 side the capillaries, and partly to the greater pressure 

 put upon the liquid inside. That this last is one of 

 the causes is proved by the phenomena of dropsy — a disease 

 in which the exudation is unduly rapid. Dropsy in the legs 

 gets worse during the day, when by sitting and standing the 

 weight of the blood to be borne by the vessels of the legs is 

 increased ; and gets better during the night, when by the 

 recumbent attitude these vessels are relieved from this 

 weight. Contrariwise, that cedematous swelling under the 

 ej^es which is common in the aged and debilitated, increases 

 during the night and decreases during the day — gravitation 

 serving, when the body is upright, to diminish the pressure of 

 the blood at this part, and not having this effect when the 

 body is horizontal. But if the plasma is to some extent 

 forced through the walls of the capillaries by pressure, then 

 not only will the action of the heart, aided at some parts by 

 gravity, further the exudation, but the exudation will be 

 furthered by external pressures from time to time falling on 

 the capillaries. If the capillaries of the skin be squeezed 

 by the thrust of some object against the surface, part of their 

 contained blood will be driven back into the arteries, more 

 will be driven forwards into the veins, and some will be made 

 to exude. Immediately they are relieved from the pressure 

 they will be refilled from the arteries, again to yield an extra 

 portion of their contents to the tissues around when again 

 squeezed. Thus recurrent thrusts or impacts, acting on the 

 body from without, aid in the nutrition of the parts on which 

 they fall : producing, in some cases, a node upon the subjacent 

 bone, as on the instep where a boot has pinched ; producing, 

 in other cases, growth of the connective tissue, as in a bunion ; 



