510 MATERIA MEDIC A AND THERAPEUTICS. 



glomerulus by the afferent vessel, and (2) by the resistance to 

 its flow through the efferent vessel ; whilst the freedom of 

 filtration depends upon the fact that the uriniferous tubules 

 have a free outlet, and thus present but little obstruction to the 

 entry of water into their channel. 



The size of the renal vessels is regulated by vaso-motor 

 nerves, coming chiefly from the splanchnics, which derive their 

 renal fibres from the medulla oblongata, in part at least through 

 the first thoracic ganglion. The spot in the fourth ventricle 

 which thus presides over the vessels of the kidney is a centre, 

 i.e. it receives impressions through afferent nerves, and sends 

 impulses through efferent nerves to the kidneys. Thus powerful 

 emotions will disturb the flow of urine, and the temperature 

 of the surface of the body affects the amount of urine secreted, 

 partly at least reflexly. 



The solid constituents of the urine urea, uric acid, and their 

 allies, and many of the salts, dissolved of course in a small 

 quantity of water are probably separated from the blood by 

 the cells of the convoluted tubules. The activity of the renal 

 epithelium no doubt depends, like that of the salivary glands, 

 upon an inherent secreting force of its own, probably con- 

 trolled by trophic nerves ; upon the activity of the circula- 

 tion ; and especially upon the quality of the blood. "We have 

 already seen that the materials which the blood conveys to the 

 kidney for excretion will depend upon the activity of all the 

 bodily functions, and we will not return to this subject except 

 with respect to the influence of digestion and assimilation on the 

 urine. During gastric digestion a quantity of acid is withdrawn 

 from the blood to furnish the gastric juice, and this loss of 

 acidity in a fluid already alkaline makes itself felt in the urine, 

 which soon becomes less acid, or even alkaline. This reaction 

 increases when absorption begins. Water and salts enter the 

 blood ; augment still further the alkalinity of the urine, the 

 salts being chiefly alkaline, and the total volume of the blood, 

 and thus of the renal secretion, is increased ; the arterial 

 pressure rises. Finally, the products of the action of the liver, 

 lungs, and other metabolic organs, upon the peptones and 

 carbohydrates (urea and its allies) also enter the blood and 

 appear in the urine, in comparative excess. This condition of 

 the urinary function and urine, consequent on a full meal, 

 gradually declines. The excess of water escapes ; the alkaline 

 salts are voided ; the excess of urea and uric acid disappears ; 

 and therewith the general characters of the urine change. 

 By the end of three or four hours from the admission of food, 

 the urine is again moderate in amount, more acid, and 

 clear, an increase of acidity following the previous reduction. 



