REMOVAL OF URINE SALTS. 223 



When the substance administered is of such a chemical nature that it 

 can unite with any tissue, it may remain in the system for a very long 

 time. 



The anatomical construction of the Malpighian bodies has led physi- 

 ologists to infer that there are two distinct stages in the se- Manner of se- 

 cretion of urine. These have already been pointed out in jfiJlto ij 

 the remark that the Malpighian bodies separate water from filtration, 

 the blood, but that the solid ingredients are secreted from that delicate 

 plexus of vessels which covers the walls of the urinary tubes. Before 

 accepting this opinion, we may, however, observe, that the chief solid con- 

 stituents of the urine, as urea, uric acid, sulphates, and phosphates, pre- 

 exist in the blood, and are all soluble in water. It is not to be supposed 

 that the water which oozes through the delicate walls of the Malpighian 

 tufts should leave such substances behind it. That the loss of water 

 actually takes place in the tuft circulation appears to be proved by the 

 fact that the vessel emerging from the tuft is less than the one entering 

 it ; the volume of blood is less by the amount of abstracted water. 



We must, moreover, take care that we are not deceived by a name. 

 The vessel emerging from the tufts may be conveniently ThQ arterial 

 enough called a vein, but is there any proof that such is its quality retain- 

 physiological attitude? ' There is no reason to believe that ed in the tufts ' 

 the blood has lost its arterial character while it has been in the tuft. At 

 the most, it can only have lost the elements of urine. It is not until it 

 is distributed in the plexus on the walls of the uriniferous tubes that it 

 really gains the venous character, and then through nourishing those ves- 

 sels, and particularly the cells of their interior. 



These considerations therefore lead me to the suggestion that the inor- 

 ganic bodies, as urea, uric acid, sulphates, and phosphates, which may all 

 be regarded as products of final oxidation, pass out with the water in 

 which they are dissolved while the blood is yet circulating in the Mal- 

 pighian tuft. The loss of velocity in the current by the arterial twig 

 breaking up into so many vessels must, as Mr. Bowman states, greatly 

 favor this transudation, as does also the pressure that must arise, from the 

 blood having to pass through a narrow channel of exit, and still more 

 through another capillary system just beyond. It was arterial blood that 

 entered the tuft, and it is arterial blood that emerges, to be then directed 

 upon the walls of the uriniferous tubes. 



And now the question may arise, What is the object of this second cap- 

 illary circulation ? Though the statement is often made that The cells re- 

 the. constituents of the urine are the results of oxidation, it ^Jdw^" 

 is very far from being strictly true. The analysis of urine stances, 

 shows that a very large proportion of them, classed as extractive, are real- 

 ly combustible bodies, and not far advanced in their retrograde meta- 



