72 PHYSIOLOGICAL EEGULATIONS 



sugars (table 4, column 5). By various means water exchanges in 

 the diverse types of positive and negative load may be abolished 

 altogether, or may be tripled in rate (tables 3 and 4, colmnns 7), 

 as compared with those prevailing in the two types of water loads 

 of § 7 and § 8. 



A few criteria are suggested for classifying difficult types into 

 positive loads and negative loads. The relative rates of various 

 water exchanges during recovery themselves usually serve as cri- 

 teria of the direction and amount of disturbance in water content. 

 Sometimes unusual paths of water exchange aid in adjusting the 

 contents. 



The content of water at balance is also modified by many agents. 

 The rates of exchanges (turnover) that prevail at balance may or 

 may not suffer modification at the same time. Each state of the 

 dog with respect to water therefore calls for characterization both 

 in regard to water content and in regard to modifications in rates 

 of water exchange, as greater and smaller loads of both signs are 

 present. 



In the story of regulations of water content, there could be a 

 chapter describing the behavior of the dog toward water in the 

 environment. What acumen does the dog show in finding water 

 and avoiding water? Does it hasten its water gain and minimize 

 its water loss by choosing appropriate surroundings when it is in 

 water deficit? Unfortunately for the present enterprise, no quan- 

 titative information is available in answer to those questions. This 

 particular method of regulation has been studied especially in the 

 rat (§43) and in insects (§47). 



Qualitatively there is much evidence that dogs, like most other 

 animals, seek out and stay in environments that favor their main- 

 tenance of water content. They avoid desert areas, they system- 

 atically search for sources of water supply. In other words, they 

 use sensorimotor abilities to evade serious difficulties in supplying 

 themselves with water, and in surrounding themselves with a para- 

 dise of water. By such choices they forestall the frequent use of 

 compensations on any large scale; this is a prevention that pre- 

 cludes the need for cure. Behavior can be thought of as a sepa- 

 rate line of defense against water loads, modifications of water 

 exchange as an insurance when behavior fails. More strictly both 

 are coordinate and specific means of maintaining and recovering 

 water content. 



