GENERAL CONSIDERATIONS. 711 



health, Ave must of necessity, in regarding the state of the 

 urine as an index of disease, allow to it a considerable range or 

 scope, both as to quantity voided over a determinate period, and 

 the relative amount of tlie several materials. Above or below 

 these points, we may agree to view it as indicative of con- 

 ditions apart from health. 



Without minutely entering into the chemistry of the urine, 

 as evidenced apart from or even in connection with disease, 

 we may regard it as exhibiting, in over ninety per cent, of 

 water, a solution or mixture variously arranged of a rather 

 important and complex nitrogenous compound, urea: two 

 acids, uric and hippuric ; a considerable amount of inorganic 

 salts ; several organic substances of a nature rather ill under- 

 stood, known as extractive matter ; and certain colouring prin- 

 ciples. 



These materials represent the various exchanges, or rather 

 results of the various exchanges, taking place between certain 

 tissues or elements of the body, and^varying extrinsic agents, 

 together with the regular removal of particular elements which 

 have served their purpose, or which have been developed in 

 the course of the activities taking place. 



Conditions which Modify the Urinary Secretion. — From what 

 we know of the structure and action of the kidneys, we can 

 easily understand that any disturbance in the blood-flow 

 through the capillaries of the malpighian tufts, its increase or 

 diminution in any given time, as well as any alteration in that 

 fluid which renders its liquid parts more or less likely to pass 

 readily through animal membranes, or which confer upon it a 

 greater solvent j^ower upon materials wherever placed, and 

 which require removal through the kidneys, must largely 

 modify the amount and quality of the secretion. 



This condition of the blood-channels and quality of their 

 contents is largely influenced by conditions other than changes 

 situated in the glands themselves. Such disturbing influences 

 we observe ever at work in many general congestions origi- 

 nating from apparently extrinsic influences, or through the 

 development of indwelling morbitic agencies. Such are 

 exemplified in sudden chills or lowering of the surface-tem- 

 peratures, with disturbance of the circulation in many large 

 and vascular organs, and as attendant upon hremal vitiation. 



