660 THE SECRETION OF URINE. 
tubules of the kidney suggests that in this organ, as in the salivary 
glands, the secretion of urine may he under the direct control of the 
central nervous system, apart from any influence that this system may 
have on the renal circulation. We have already seen that the urinary 
secretion is extremely susceptible to variations in the pressure and 
velocity of the blood in the renal vessels, and also that these latter are 
under the direct control of the nervous system by means of vaso-dilator 
and vaso -constrictor nerve fibres. 
Various authorities have described experiments which should 
demonstrate the existence of secreto-motor nerves to the kidney. Thus, 
in 1835, Claude Bernard 1 showed that in some cases, where puncture of 
the medulla was carried out with the view of producing diabetes, the 
result was an increased flow of urine, containing no sugar, i.e. diabetes 
insipidus. These experiments were repeated in much greater detail by 
Eckhard, 2 who showed that, in the rabbit, polyuria might be caused, not 
only by a puncture of the medulla, but also by chemical or mechanical 
stimulation of the neighbouring portion of the superior vermis of the 
cerebellum, especially if, previously to the operation, the nerves going 
to the liver had been divided. Moreover, it is a familiar fact to 
clinicians, that injuries to the head, epileptic attacks, and especially 
lesions in the neighbourhood of the medulla, may bring about a condi- 
tion of diabetes insipidus. 
We know already that division of one splanchnic nerve will cause 
an increased secretion of urine in the kidney of the same side, and it is 
natural to imagine that the mechanism of the increased urinary sec- 
tion after the piqvure is of the same nature. Eckhard pointed out, how- 
ever, that the course of events is different in the two cases. After division 
of one splanchnic, the flow of urine is almost immediately somewhat 
increased, and this moderate increase lasts a considerable time (three to 
four hours at least). The first result of puncture of the medulla is a 
cessation of the urinary How. This is followed shortly by an increase 
much greater than is caused by section of the splanchnic, but only 
lasting one to two hours. Moreover, the effect of the diabetic puncture 
is observable even after section of the splanchnics, as well as of all the 
nerves which may possibly send branches to the kidney. Eckhard con- 
cludes, therefore, that the effect must be due to one of two causes: 
either an increased general blood pressure, in consequence of the 
stimulus caused by the puncture, or the excitation of nerve fibres which 
run in the walls of the renal artery itself. The first explanation must 
lie rejected, since direct measurement of the 1 flood pressure does not 
show any definite rise in consequence of the puncture. We must there- 
fore accept the second explanation as the correct one. Eckhard regards 
these nerve fibres as secreto-motor, and believes that the urinary secre- 
tion is under the control of a nerve centre, situated most probably in 
the medulla. He bases this hypothesis on the facts that section of the 
cord below the medulla stops the flow of urine, and that stimulation i if 
the cut cord does not bring back the flow, in spite of the rise of blood 
pressure which is induced. We know now, however, that the negative 
result of stimulating the cut cord is due to the constriction of the renal 
vessels, which occurs together with those of other parts of the body, so 
1 " Lecons de physiol.," 1835, tome i. p. 339. 
- Beltr. :.. Anat. u. Physiol. (Eckhard), Giessen, 1869, Bd. iv. S. 1-32 and 153-193; 
1870, Bd. v. S. 147-178 ; 1872, Bd. vi. S. 1-18 and 51-94. 
