CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 531 



are supplied from what we have previously ( 147 and elsewhere) 

 spoken of as the vaso-con stricter 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 6th thoracic, a few perhaps 

 even to the 4th thoracic, and as low down as the 2nd lumbar 

 (4th lumbar if only 13 nerves be counted as thoracic) ; but most 

 seem to pass by the llth, 12th and 13th thoracic nerves. Passing 

 through the corresponding ganglia of the sympathetic chain, 

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

 the splanchnic nerve ; those however coming from some of the 

 lower nerves apparently do not contribute to the splanchnic 

 nerve, but take a separate course. Centrifugal stimulation of 

 these anterior roots produces shrinking of the kidney, all the 

 more marked and distinct in the case of the llth, 12th and 13th 

 thoracic roots because the effect on the kidney is then not so much 

 masked by vaso-motor effects on other organs. Stimulation 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. 



333. There is also some evidence gained by the method 

 of slowly repeated rhythmical stimulation ( 146) that some of 

 the higher (anterior) roots also contain renal vaso-dilator fibres ; 

 but the matter is not at present beyond dispute. 



334. It is obvious then that by means of this vaso-motor 

 mechanism the flow 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 

 dilation 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 changes in the flow of blood through the kidney, 

 as shewn by changes of volume, may be brought about quite 

 apart from the central nervous system. For instance after all 

 the nerves going to the kidney have been severed, the kidney, 

 as shewn by the oncometer, swells when substances such as urea, 

 which cause an increase in the secretion of urine, are injected 

 into the blood. The substance reaching the kidney by the blood 

 stimulates the kidney to activity, and this is accompanied by a 

 dilation of the blood vessels, which, since the nerves have been 

 severed, must be brought about by some local action. The event 

 seen is similar to the greater flow of blood through a muscle 

 when it contracts. Cf. 146. 



