416 THE ELIMINATION OF WASTE PRODUCTS. 



are supplied from what we have previously ( 155 and elsewhere) spoken of 

 as the vaso-constrictor region of the spinal cord. They issue from the spinal 

 cord by the anterior roots of a large number of the spinal nerves taking 

 origin from this region, and may be traced (in the dog) as high up as the 

 sixth dorsal, a few perhaps even to the fourth dorsal, and as low down as the 

 second lumbar (fourth lumbar if only thirteen nerves be counted as dorsal) ; 

 but most seem to pass by the eleventh, twelfth, and thirteenth dorsal nerves. 

 Passing through the corresponding ganglia of the splanchnic (sympathetic) 

 chain, these fibres reach the solar plexus and thus the renal plexus by the 

 abdominal splanchnic nerve ; those, however, coming from some of the lower 

 nerves apparently do not contribute to the splanchnic nerve, but take a sepa- 

 rate course. Centrifugal stimulation of these anterior roots produces shrink- 

 ing of the kidney, all the more marked and distinct in the case of the 

 eleventh, twelfth, and thirteenth dorsal roots because the effect on the kidney 

 is then not so much masked by vasomotor effects on other organs. Stimu- 

 lation of the higher roots also produces shrinking of the kidney, but less 

 marked, since in these cases the stimulation bears at the same time largely 

 on vaso-constrictor fibres for other abdominal organs, and so by raising the 

 general blood-pressure tends to neutralize the local effect on the kidney. 

 And even the very decided shrinking of the kidney which results from the 

 stimulation of the splanchnic trunk itself is less than would take place if 

 the stimulation affected the vessels of the kidney only. 



349. We stated in 154 that by the method of slowly repeated rhyth- 

 mical stimulation the presence of vaso-dilator fibres in the sciatic nerve 

 might be detected, though these are largely mixed with vaso-constrictor 

 fibres ; and slow rhythmical stimulation of the anterior roots of the above- 

 mentioned lower dorsal nerves leads, not, as does ordinary rapidly inter- 

 rupted stimulation, to shrinking, but to swelling of the kidney, showing that 

 these roots contain vaso-dilator fibres as well as vaso-constrictor fibres. The 

 higher (anterior) roots also appear to contain some renal vaso-dilator fibres ; 

 but the effect of stimulating them by the slow rhythmic method is more 

 masked by a concomitant dilatation of the vessels of the other abdominal 

 organs, the roots in question containing vaso-dilator as well as vaso-con- 

 strictor fibrjes for those organs ; this leads to a fall of general blood-pressure 

 whereby the tendency of the kidney to swell is counteracted. 



350. It is obvious then that by means of this vasomotor mechanism 

 the ffow of blood through the kidney is governed by the central nervous 

 system in such a way that afferent impulses, started in this or that region or 

 surface, and passing up to the central nervous system, may lead either to 

 constriction or to dilatation of the renal vessels ; and to such actions of this 

 kind we shall presently return. Meanwhile we wish to call attention to the 

 fact that the volume of the kidney is remarkably sensitive to chemical 

 changes taking place in the blood. The injection into the blood of even a 

 small quantity of water causes a transient shrinking of the kidney followed 

 by a more lasting expansion. The injection of urea and some other diuretics 

 produces the same effect to a more marked degree, leading especially to a 

 swelling which lasts for some considerable time, while the injection of normal 

 saline solution, and especially of such diuretics as sodium acetate, causes an 

 expansion from the very first, the primary shrinking being absent. It is, 

 moreover, worthy of note that these effects of diuretics and of chemical 

 changes in the blood are observed even after all the renal nerves have 

 apparently been completely severed. Hence the changes in volume caused 

 by the presence of these substances in the blood must be due to the sub- 

 stances acting either upon some peripheral vasomotor mechanism, or, even 

 more directly, on the bloodvessels themselves. It may be added that they 



