ACTION OF DIURETICS. 511 



II. PHARMACODYNAMICS. 



The preceding considerations prepare us for the conclusion 

 that what power we may possess over the excretion of urine 

 will be exercised, as far as its water is concerned, chiefly through 

 the circulation ; as far as the solids are concerned, chiefly 

 through the hlood. These points must he separately studied. 



1. Measures for Increasing the Volume of Urine. The amount 

 of water, that is, the volume of urine which is excreted from 

 the glomerulus, may he increased hy diuretics, the effect being 

 called diuresis (Sia through and ovpov the urine). This may be 

 accomplished in various ways : 



(a) By raising the pressure in the arteries generally, including the 

 renal, whilst the pressure in the veins is constant. This is 

 most easily effected by temporarily increasing the amount of 

 water in the system by drinking ; by raising the force or the 

 frequency of the heart, or both, by Alcohol, Digitalis, Scilla, 

 Ammonia, and Scoparium ; or by constricting the peripheral 

 vessels through the vasomotor system, e.g. by cold to the 

 surface, Digitalis, Scilla, or other vascular stimulants. These 

 measures are called cardio-vascular diuretics. 



(b) By dilating the renal arteries, so that the quantity 

 of blood within them is increased, whilst the pressure in 

 the arterial system generally, and the resistance in the 

 renal veins, remain unchanged. This method of increasing 

 the amount of the renal water may be carried out by 

 acting on the vaso-motor system of the kidney either 

 locally or centrally. Local depressants of the renal nerves 

 include Digitalis and Scilla in the second stage; Spirit of 

 Nitrous Ether ; all Volatile Oils and Eesins, such as Turpentine, 

 Juniper, Copaiba, Hops, Savin, Cantharides, Camphor, etc. ; 

 Alcohol, Belladonna, Aconite, Nitrates, and Nitrites. Central 

 renal vascular depressants are chiefly or solely emotional impres- 

 sions which are not available as pharmacodynamical means. A 

 powerful reflex dilator of the renal vessels is cold to the surface. 

 Such measures are local vascular diuretics. 



(c) By combining the two previous means, when still more 

 profuse diuresis will be the result. This occurs in the 

 second stage of the action of Digitalis and Scilla, and in the ap- 

 plication of cold to the surface. 



2. Measures forDiminishing the Volume of Urine. The volume 

 of urine might be diminished by employing the opposite set of 

 influences to those just described. These are obscure, however, 

 and of less therapeutical interest ; and the student may be left 

 to work out the different systems for himself. 



3. Measures affecting the Secretion of Urinary Solids. The 



