53 8 THE URINARY APPARATUS. [CH. xxxvi. 



there is in addition to this what is usually termed the filtering 

 apparatus : we have already seen that the tufts of capillary blood- 

 vessels called the Malpighian glomeruli are supplied with afferent 

 vessels from the renal artery ; the efferent vessels that leave 

 these have a smaller calibre, and thus there is high pressure in 

 the Malpighian capillaries. Certain constituents of the blood, 

 especially water and salts, pass through the thin walls of these 

 vessels into the surrounding Bowman's capsule which forms the 

 commencement of each renal tubule. Though the process which 

 occurs here is generally spoken of as a filtration, yet it is no 

 purely mechanical process, but the cells exercise a selective 

 influence, and prevent the albuminous constituents of the blood 

 from escaping. During the passage of the water which leaves the 

 blood at the glomerulus through the rest of the renal tubule, it 

 gains the constituents urea, urates, &c., which are poured into it 

 by the secreting cells of the convoluted tubules. 



The term excretion is better than secretion as applied to the 

 kidney, for the constituents of the urine are not actually formed 

 in the kidney itself (as, for instance, the bile is formed in the 

 liver), but they are formed elsewhere ; the kidney is simply the 

 place where they are picked out from the blood and eliminated 

 from the body. 



The Nerves of the Kidney. 



Nerves. The nerves of the kidney are derived from the renal 

 plexus of each side. This consists of both medullated and non- 

 medullated nerve-fibres, the former of varying size, and of nerve- 

 cells. Fibres from the anterior roots of the eleventh, twelfth, and 

 thirteenth dorsal nerves (in the dog) pass into this plexus. They 

 are both vaso-constrictor and vaso-dilatator in function. The 

 nerve-cells on the course of the constrictor fibres are situated in 

 the cceliac, mesenteric, and renal ganglia ; the cells on the course 

 of the dilatator fibres are placed in the solar plexus and renal 

 ganglia. 



These nerves are thus vaso-motor in function ; we have at 

 present no knowledge of true secretory nerves to the kidney ; the 

 amount of urine varies directly with the blood-pressure in its 

 capillaries. 



Increase in the quantity of urine is caused by a rise of intra- 

 capillary pressure. This may be produced by increasing the 

 general blood-pressure ; and this in turn may be produced in the 

 following ways : 



(i.) By increase in the force or frequency of the heart beat. 



(2.) By constriction of the arterioles of areas other than that 



