§ 5.321 WATER BALANCE 225 



hormone is stimulated nervously from the ganglionic network 

 round the gut; if this is damaged, or if the circumocsophagcal 

 connectives to the brain are cut, hormone secretion is not induced 

 when the abdomen begins to swell. This nervous stimulation of 

 hormone secretion recalls the activation of the pigment-controlling 

 hormone of Caraiisius (§ 3.221), but is unusual for a metabolic 

 hormone (§ 5.522), unless it be for the neurohypophysial hormones 

 (§ 5.322). 



Vertebrata. Hydrocortisone*, from the adrenal cortex 

 (§ 2.31), has been found to have a diuretic effect on the kidneys, 

 and also on the skin, of some vertebrates ; but the evidence is most 

 definite for amphibians and mammals (Table 25). 



Amphibia. When amphibians enter their normal freshwater 

 environment, excess water tends to enter through any per- 

 meable tissues and to dilute the salt concentration in the 

 blood. 



No observations on the action of hormones on the skin have so 

 far been recorded in connection with limiting this endosmosis ; it 

 would clearly be of interest to know whether the relative imper- 

 meability that is usual for the skin (controls. Fig. 5-20 a-c^) results 

 merely from the lack of antidiuretic hormone, (§ 5.322), or whether 

 a positive, pore-contracting action of an adrenocortical hormone 

 may also be involved. The similarity in behaviour between isolated 

 skin and that in whole animals would lend no support to the latter 

 supposition, unless hydrocortisone persists for a long time in the 

 tissues after isolation. 



The reactions are slightly better known in the kidney, where 

 diuresis seems to be facilitated by hydrocortisone. On exposure 

 to a hypotonic medium, salts are actively reabsorbed in the proxi- 

 mal tubules (§ 5.311). This would result, as in the mammalian 

 kidney (Fig. 5- 15a, p. 214) in an obligatory endosmosis of water. 

 In so far as the inward diffusion of water failed to keep pace with 

 the transport of the salts, 'Tree water" would remain in the 

 tubules, converting the isosmotic glomerular fihrate into hypotonic 

 urine. The greater the degree of impermeability to water possessed 

 by the kidney tubules, the greater would be the hypotonicity of the 

 urine. There is no clear evidence as to how this impermeability is 



* Several related 17-OH steroids have the same effect (Part II). 



