410 OF SECRETION. 



of fulness of the blood-vessels; whilst the amount of solid matter is 

 governed, as we shall presently see, by the previous waste of the tis- 

 sues. The quantity of fluid in the blood-vessels is governed by the 

 relative amount that has been absorbed, and that which has been 

 exhaled from the skin ; so that the quantity to be drawn off by the 

 Kidneys is increased, either by augmented absorption, or by diminished 

 exhalation. The Malpighian bodies seem to act the part of a system 

 of regulating valves ; permitting the transudation of only enough fluid 

 to dissolve the solid matter, when there is no superfluity of water in 

 the vessels ; but allowing the escape of an almost unlimited amount of 

 it, when increased imbibition has rendered the vessels unusually turgid. 



729. The average amount of Urine excreted in twenty-four hours, 

 by adults who do not drink more than the wants of nature require, is 

 probably from 30 to 40 oz. ; and its average specific gravity may be 

 about 1020. The quantity of fluid is usually less, and the specific 

 gravity of the secretion consequently greater, in summer than in win- 

 ter, on account of the larger proportion of fluid exhaled by the skin 

 during the former season. The quantity of solid matter has been found 

 to vary, within the limits of ordinary health, from 3-6 to 6'7 per cent.; 

 and the extent of variation in disease is doubtless much greater. About 

 one-third of the solid matter is made up of alkaline and earthy salts ; and 

 the remainder is made up of organic compounds. The salts are partly 

 those of the blood, which will not be separated during the transudation 

 of the serum through the membranous walls of the Malpighian capilla- 

 ries, although the albuminous matter is kept back ( 196). But there 

 is a much larger proportion of the alkaline and earthy phosphates in 

 the urine, than is present in the blood ; and this is liable to a further 

 increase under circumstances to be presently alluded to. The urine is 

 normally acid, but the degree of its acidity has been shown by Dr. 

 Bence Jones to be continually changing, and to be considerably affected 

 by food ; being augmented by vegetable, and decreased by animal food. 

 What the acid may be to which the acidity is due, is yet uncertain ; 

 possibly it is not always the same. 



730. The organic compounds present in the Urinary secretion (in its 

 healthy state at least.) are undoubtedly the result of the waste or disin- 

 tegration of the animal fabric ; as well as (in certain cases) of the de- 

 composition of constituents of the blood, which have never undergone 

 conversion into organized tissue. Their unfitness to be retained* within 

 the system, is proved by the fatal results which speedily ensue when 

 their elimination by the secreting process receives a check ; and also 

 by the crystalline form, in which the most characteristic of them pre- 

 sent themselves, such a form being altogether incompatible with the 

 possession of plastic or organizable properties. Various well-defined 

 compounds present themselves in the Urine of different classes of ani- 

 mals ; and they are nearly all peculiarly rich in Nitrogen and deficient 

 in Carbon, as compared with the Albuminous compounds. Thus, whilst 

 the proportion of Nitrogen in Albumen is (by weight) as 1 : 6*24 of the 

 whole, it is in Urea as 1 : 2-14, in Allantoin as 1 : 2*21, in Kreatine 

 as 1 : 2-69, in Uric acid as 1 : 3-00, and in Kreatine as 1 : 3-12. The 

 only exception is in the case of Hippuric acid, which is discharged 



