500 OF SECKETION AND EXCRETION. 



410. The Specific Gravity conies to be a very important character in vari- 

 ous morbid conditions of the urine ; and it is therefore desirable to estimate 

 it correctly. This also is of course subject to the like causes of variation ; 

 since, when the same amount of solid matter is dissolved in a larger or 

 smaller quantity of water, the specific gravity will be proportionably lower 

 or higher ; or, the quantity of water remaining the same, an increase or dimi- 

 nution in the amount of solid matter will raise or lower the specific gravity. 

 It has been commonly supposed that the amount of solid matters in the 

 urine bears such a constant ratio to its specific gravity, that the former may 

 be approximately deduced from the latter; this, however, is now ascertained 

 to be by no means the case. 1 Still, the determination of the specific gravity 

 is of sufficient importance for diagnostic purposes, to make it desirable to 

 possess an average standard, as nearly approaching to accuracy as circum- 

 stances will permit. The average, according to Dr. Prout, in a healthy 

 person, taking the whole year round, is about 1020 ; the standard rising in 

 summer (on account of the greater discharge of fluid by perspiration) to 

 1025 ; and being lowered in winter to 1015. Simon, however, states the 

 average specific gravity at no more than 1012. Dr. Roberts 2 observed the 

 specific gravity of his own urine, while in a healthy state, to vary from 1001 

 to 1036. Pollack 3 found that in infants of a few weeks old it varied from 

 1005-1007. That the specific gravity does not bear any constant relation 

 to the quantity discharged, has been shown by Dr. Hammond ; who noticed 

 that, although the afternoon and evening urine was most abundant, its spe- 

 cific gravity was very high, in consequence of its containing a large pro- 

 portion of solid ingredients. The specific gravity of the urine probably 

 depends mainly, in each individual case, upon the amount of azotized solids 

 and of aqueous fluids habitually ingested, allowing for the portion of the 

 latter that is dissipated by cutaneous exhalation ; and it will also vary with 

 the period that has elapsed since the last introduction of liquid into the 

 stomach. From these and other causes, the amount of solid matter in 1000 

 parts of urine may vary from 20 to 70 parts ; and hence the various re- 

 corded analyses of this liquid present very wide diversities in the propor- 

 tion of its solid constituents. 4 These discrepancies, however, being chiefly 

 due to the fluctuating amount of water, become very much less (as Simon 

 pointed out) when we calculate the proportion which each principal com- 

 ponent bears to 100 of solid residue ; as is shown in the following table : 



1871, vol. i, p. 6) attributes the congestion and haemorrhages seen in the kidneys and 

 other abdominal organs to contraction of the veins, or to simultaneous contraction 

 of both arteries and veins. Ollivier (Archives de Medecine, Feb. 1874) gives various 

 chemical observations showing the rapidity with which albumen appears in the urine 

 after an attack of apoplexy. 



1 Si-e Lehmann's Physiological Chemistry (Cavendish Society), vol. ii, p. 436. 



2 Kdinb. Med. Journ., 1860. 



3 Loo. cit 



4 It is remarked by Lehmann (op. cit,, p. 447), that the urine of the French is 

 poorest in solid constituents, especially in urea and uric acid, and that of the English 

 the richest, that. <>f the Germans being intermediate between the two ; the ratio in 

 each nation being in conformity with the proportion of animal food entering into its 

 ordinary diet. 



