92 PHYSIOLOGY. 



much convoluted upon themselves, Hke a ball of twine, into which 

 a small afferent branch of the renal artery is seen to enter, and an 

 efferent venous twig to emerge, the points of immergence and emer- 

 gence being very near to each other. Each of these Malpighian cor- 

 puscles is included in a flask-like expansion of one of the lubuH 

 uriniferi which is dilated to receive it. It is at this point, and from 

 the arterial blood of the corpuscle, that the vxUery parts of the urine 

 are separated by soaking through the walls of the blood-vessels and 

 tubes, the blood being detained in the corpuscles for this purpose. 

 The venous twig after leaving the corpuscle of Malpighi, becomes 

 again capillary, and interlaces with the uriniferous tubes at another 

 point ; and at tJiis point, and from venous blood, as in the vena portce, 

 the solid matters of the urine are separated by the agency of cells 

 developed in the walls of the tube. (Fig. 23.) 



It would thus seem that the function of the Malpighian corpuscles 

 is to get rid of the superfluous water in the blood, which, from their 

 peculiar arrangement, they are well calculated to do. In this respect 

 they assist the skin, which is very liable to great variations in the 

 amount of fluid it exhales from the temperature of the air around. 



The quantity of solid matter eliminated by the kidney has no 

 reference to the amount of water to be got rid of; being dependent 

 upon the amount of waste in the system, and upon the quantity of 

 surplus azolised aliment which has to be discharged through this 

 channel. 



The average quantity of urine voided in twenty-four hours, by 

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

 from 30 to 40 oz. Its average specific gravity is 1020. The amount 

 voided is less in summer than in winter, on account of the large 

 exhalations from the skin in the former season. The quantity of 

 solid matter varies in health from 3-6 to 6*7 per cent. About one- 

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

 the rest consists of organic compounds. 



The urine in health usually presents an acid reaction; this de- 

 pends, however, upon certain conditions furnished by the aliment, 

 and may be altered by a change in the ingesta. 



The most important constituent of the solid matters is wrm, a 

 transparent crystalline substance, soluble in water, and combining 

 with acids without neutralising them. In chemical composition it is 

 identical with cyanate of ammonia, viz.: 2 carb., 2 ox., 2 nit., 4 

 hyd. The amount of urea excreted may serve as a measure of the 

 waste of the system, especially of the muscular system. The 

 average amount of urea is about 30 parts in 1000 of urine. The 

 amount excreted by children is much greater in proportion to their 

 bulk than in old men. This corresponds precisely with the rapidity 

 of interstitial change at different periods of life. 



JJric or lithic acid is another important ingredient. It exists in 



