THE CAU^EH <>E EDEMA 347 



feet the eolloitl-s in a way similar to acids, or tlirougli the conversion 

 of colloids M^hieh have but little affinity for water into such as have 

 a fjfreater affinity, must also be bonie in mind." In support of this 

 theory he advances evidence which lie interprets as indicatin*^ that: 

 (1) "xVn abnormal production or accumulation of acids, or condi- 

 tions predisposing thereto, exist in all states in which we encounter 

 the development of an edema. (2) The development of an edema in 

 tissues is antagonized by the same substances which decrease the 

 affinity of the (hydrophilic) enuilsion colloids for water (salts) and is 

 unaffected by the presence of substances which do not do this (non- 

 ■eleetrol5i;es). (3) Any chemical means by which we render possible 

 the abnormal production or accumulation of acids in the tissues is 

 accompanied by an edema." 



There are many features of lymph formation and edema with 

 which this theory seems to harmonize well, and others with w'hich it 

 •does not seem to agree so well, if at all, so that at this time it is a 

 fair statement that the theory is under consideration, but the limita- 

 tions of its applicability have not yet been agreed upon. It has met 

 with much adverse criticism, some of w^hich was poorly founded, but 

 the fact cannot be disputed that the amount of water that colloids 

 will hold varies greatly with changes in the colloids. "We may not 

 T\now absolutely, at present, whether the changes that take place in 

 the colloids during life are great enough to alter tlieir water content 

 appreciably, but it is highly probable that they are. In many in- 

 stances the principles of colloidal hydration offer the best explanation 

 ■of observed conditions, and their application often elucidates matters 

 more satisfactorily than any other working hypothesis. Certainly 

 they cannot be disregarded in considering the factors that may come 

 into iilay in producing edema. 



Summary. — We find that a number of factors may be considered 

 as responsible for edema, some of them being prominent in one in- 

 stance, some in another, but in feiv cases can we consider one factor 

 alone as the sole cause. In most of the forms of edema, such as those 

 due to renal disease and cardiac disease, it now seems probable that 

 either osmotic pressure changes or changes in the affinity of the tissue 

 <3olloids for water, play the most important part ; whereas in inflamma-\ 

 tory edema there can be no question that alteration in the capillary 

 ■walls is the most essential factor. But the mechanical factor of blood 

 pressure cannot be disregarded, although by itself seldom sufficient to 

 cause edema ; associated with other factors it is undoubtedly an im- 

 portant agency, for there are few edemas that are not associated with 

 increased blood pressure. Hydremia and hydremic ])lethora may al- 

 most he disregarded, except in so far as they may cause altered metab- 

 olism in the tissues, injuiy to vessel-walls, over-saturation of the blood 

 ■colloids, and decreased osmotic pressure within the vessels. Lymph- 



