Chapter VIII 



EQUILIBRATIONS IN PARTS OF ORGANISMS 



§ 57. The regulation of water content has now been examined 

 in whole organisms. The individual is a convenient unit for study 

 in that it usually maintains itself in a semi-isolated condition. The 

 question arises: whenever any portion of an individual can be 

 tested, are the same sorts of relations between its water exchanges 

 and water contents found as in the whole body? Such portions 

 (living units) are organs, limbs, tissue masses, cells. Do blood 

 volumes, and fibroblast sizes, also tend to be constant, and are they 

 corrected after disturbances of them? 



Equally well, aggregates of individuals might serve as units. 

 They may be the populations of nests, households, farms, herds, 

 towns, forests, continents. Nor may it be assumed that the group 

 is the mere sum of its individual units ; for when associated, new 

 conditions impinge upon the individuals. The water exchanges of 

 a whole city could be measured after droughts and after rains, after 

 interruptions of supply and after forced utilizations ; such data will 

 not be presented here. 



In the study of parts, two sorts of physiological situation may 

 be kept distinct. In one, the portion of an individual remains in 

 place {in situ) and, although loaded directly, recovers while sharing 

 its usual relations with other parts (§ 58 to <§ 62). In the other, to 

 be considered thereafter, (§63 to ^66) the portion is isolated. 



§ 58. Volumes in situ 



In the organism as a whole, the content of water was very often 

 not identified chemically, but was measured as weight, or (in proto- 

 zoa and in echinoderm eggs) as volume. In parts of individuals in 

 situ the measurement of volume becomes paramount. This may 

 be made more explicit by designating the increments by ± AV in- 

 stead of by ± AW. 



Of the volumes that are measurable in organisms, many are 

 volumes of distribution. A particular substance in known amount 

 is added to or subtracted from the body by a particular route, and 

 its increment in some tissue is subsequently ascertained. In a re- 

 stricted sense a volume of distribution is defined (Dominguez, '34) 



151 



