1922.) * Renal Portal’ System. 123 
plexus will lower the percentage of nitrogen in the urine but 
this lowered strength in nitrogen is masked by the raised 
strength due to the admixture with the nitrogen-rich venous 
agit ie Part aE Tes regards the superior chloride strength 
the ‘‘normal”’ kidney urine, since this probably has no 
Sinceman with og mixture ae the venous with the arterial 
ood, it can only be due to the increased pressure of the 
arterial blood in the asectabulat plexus (Appendix G). 
The few exceptional results whic have described 
in connection with nitrogen and chloride strengths I believe 
to be due to the belly-skin absorption factor. Though the 
absorptive area of belly-skin ! presumably pours the absorbed 
water into the anterior abdominal vein, the blood in which 
does not supply the kidneys directly, yet ‘small ventral absorp- 
tive areas of skin on the thighs are drained by factors which 
open into the renal afferent veins and so do “ supply” the 
kidneys, and it is evident that if either water or salt is ab- 
sorbed by the skin, this would reach the kidneys sooner via the 
renal afferent veins than that absorbed by the belly skin. 
Marked absorption of water will dilute the venous blood and 
when this admixes with the arterial blood in the ‘‘ normal”’ 
kidney, the result may occasionally be that the ‘‘ normal”’ 
kidney secretes urine weaker in nitrogen and chloride than 
res, 
the frog’s skin behaves as a semi-permeable membrane which 
permits water to leave or enter according to the hyper- or 
hypo-tonicity of the solutions—hence water may occasionally 
enter in great quantity. On the other hand, according to these 
authors, ‘substances, like sodium chloride, dissolved in the water, 
are always absorbed, and in the case of those frogs which 
were placed in saline during the experiment, it is not surprising 
that the “‘normal’’ kidney always secreted urine stronger in 
chloride, because the venous blood probably contained more 
chloride than the arterial blood. These are the only explana- 
tions I can offer in order to account for the few exceptional 
results which I pbteia ed. 
That the above explanation of the main results is the true 
one is proved by the results of the perfusion experiments 
ic b ose which 
hb reted by 
nitrogen strength (‘‘ 30 fluid ’’) as the arterial fluid in its renal 

This area can be well demonstrated i in Rana tigrina by im mersing 
an animal for a few — in ammonia or acid solution, when it 
assnmes a di ees red colou 
