750 PHYSIOLOGY OF DIGESTION AND SECRETION. 



directly the outflow of urine. By this and other means it has 

 been shown that the kidney receives a rich supply of vasoconstrictor 

 nerve fibers that reach it between and around the entering blood- 

 vessels. These fibers emerge from the spinal cord chiefly in the 

 lower thoracic spinal nerves (tenth to thirteenth in the dog), pass 

 through the sympathetic system, and reach the organ as postgan- 

 glionic fibers. Stimulation of these nerves causes a contraction of 

 the small arteries of the kidney, a shrinkage in volume of the whole 

 organ as measured by the oncometer (see Fig. 223), and a dimin- 

 ished secretion of urine. When, on the other hand, these con- 

 strictor fibers are cut as they enter the hilus of the kidney, the ar- 

 teries are dilated on account of the removal of the tonic action of 

 the constrictor fibers, the organ enlarges, and a greater quantity 

 of blood passes through it, since the resistance to the blood-flow is 

 diminished while the general arterial pressure in the aorta remains 

 practically the same. Along with this greater flow of blood there 

 is a marked increase in the secretion of urine. 



Under normal conditions we must suppose that these fibers are 

 brought into play to a greater or less extent by reflex stimulation, 

 and thus serve to control the blood-flow through the kidney and 

 thereby influence its functional activity. It has been shown, too, 

 that the kidney receives vasodilator nerve-fibers, that is, fibers 

 which when stimulated directly or reflexly cause a dilatation of 

 the arteries, and therefore a greater flow of blood through the or- 

 gan. According to Bradford, these fibers emerge from the spinal 

 cord mainly in the anterior roots of the eleventh, twelfth, and thir- 

 teenth spinal nerves. Under normal conditions these fibers are 

 probably thrown into action by reflex stimulation and lead to an 

 increased functional activity. It will be seen, therefore, that the 

 kidneys possess a local nervous mechanism through which their 

 secretory activity may be increased or diminished by correspond- 

 ing alterations in the blood-supply. So far is as known, this is the 

 only way in which the secretion in the kidneys can be directly af- 

 fected by the central nervous system. It should be borne in mind, 

 also, that the blood-flow through the kidneys, and therefore their 

 secretory activity, may be affected by conditions influencing general 

 arterial pressure. Conditions such as asphyxia, strychnin poison- 

 ing, or painful stimulation of sensory nerves, which cause a general 

 vasoconstriction, influence the kidney in the same way, and tend, 

 therefore, to diminish the flow of blood through it ; while conditions 

 which lower general arterial pressure, such as general vascular dila- 

 tation of the skin vessels, may also depress the secretory action of 

 the kidney by diminishing the amount of blood flowing through it. 



In what way any given change in the vascular conditions of the 

 body will influence the secretion of the kidney depends upon a num- 



