RENAL FUNCTION 255 



mid exerted large effects on the U/B inulin ratio, though it was easy to 

 demonstrate profound effects on the simultaneous excretion of other 

 materials. 



It would be of interest to understand what source of pressure might be 

 available for filtration in the cephalopod. The renal appendages are venous 

 structures lying on the main branches of the vena cava. They lie in a right 

 and a left sac, each sac having a small tubular connection to the pericardial 

 coelom surrounding the branchial heart and branchial heart appendage of 

 the same side. The tube from the pericardial coelom empties near the 

 orifice of the renal sac, making it possible that the pericardial coelom and 

 the renal sac are emptied simultaneously by a long and vigorous respira- 

 tory movement of the animal, the contents of the pericardial coelom there- 

 fore not coming into contact with the renal appendages. If this is the case, 

 our analyses of the activities going on in the renal appendages do not in- 

 clude any filtered material from the branchial hearts. Further investigation 

 of this point is in progress ; an effort is being made to collect the material 

 from the pericardial coelom quantitatively under various experimental 

 conditions. 



If we therefore exclude the branchial hearts and the branchial heart 

 appendages as a source of filtration, we must search again for a source of 

 pressure. Observations on the living animal, and on the renal appendages 

 in mtro, made by my colleague Dr. Florence M. Harrison, reveal a very 

 promising mechanism. The blood vessels of cephalopods are notably con- 

 tractile and show much rhythmic peristaltic activity. Each renal append- 

 age represents a diverticulum from a branch of the vena cava. A small 

 round or oval orifice penetrates the wall of the vena cava, leading into a 

 labyrinth of progressively finer vessels. The peristaltic movements are not 

 limited to the walls of the vena cava, but muscle tissue in the appendages 

 also contracts, compressing a considerable volume of blood in the cul-de- 

 sac represented by the appendage while the blood is forced back through 

 the aperture into the vena cava. Upon relaxation blood flows again into 

 the myriad of small vessels lined with renal epithelium. It is suggested that 

 a fraction of the fluid is forced through the walls of the appendage, where- 

 ever the lining of cells is thinnest, with each contraction. 



It is notable that in this survey of the molluscs we have now completed 

 a full cycle. In the lamellibranch the filtration pressure was thought to 

 originate in the auricular and ventricular activity of the heart and the 

 actual site of filtration perhaps to be the tissues of the heart ; in the gastro- 

 pod the filtration pressure originated in the heart but the filtration site was 

 in the capillaries and small sinuses of the arterial region ; in the cephalopod 

 the source of pressure may be contraction of muscle tissue related to that 

 of the veins and the renal appendages are situated on the walls of the veins. 



