172 HUMAN PHYSIOLOGY. 



The secretion of urine, is, therefore, partly physical and partly 

 vital. 



The filtration of urinary constituents from the glomerulus into 

 Miiller's capsule depends largely upon the blood-pressure and the 

 rapidity of blood flow in the renal artery and glomerulus. Among the 

 influences which increase the pressure and velocity may be mentioned 

 increased frequency and force of the heart's action, contraction of 

 the capillary vessels of the body generally, dilatation of the renal 

 artery, and increase in the volume of the blood. 



The reverse conditions lower the blood-pressure and diminish 

 the secretion of urine. 



The fact that organic matters are eliminated by the secretory ac- 

 tivity of the renal epithelium seems to be well established by 

 modern experiments. These substances, removed from the blood in 

 the secondary capillary plexus of blood-vessels, by a true selective 

 action of the epithelium, are dissolved and washed toward the pelves 

 by the liquid coming from the capsules. 



The blood-supply to the kidney is regulated by the nervous system. 

 If the renal nerves be divided, the renal artery dilates and a copious 

 flow of urine takes place. If the peripheral ends of the same 

 nerves be stimulated, the artery contracts and the urinary flow 

 ceases. The same is true of the splanchnic nerves, through which the 

 vaso-motor nerves coming from the medulla oblongata and spinal 

 cord pass to the renal plexus. 



LIVER. 



The liver is a highly vascular, conglomerate gland, appended to 

 the alimentary canal. It is the largest gland in the body, weighing 

 about four and one half pounds ; it is situated in the right hypo- 

 chondriac region, and is retained in position by five ligaments, four 

 of which are formed by duplicatures of the peritoneal investment. 



The proper coat of the liver is a thin but firm fibrous membrane, 

 closely adherent to the surface of the organ, which it penetrates at 

 the transverse fissure, and follows the vessels in their ramifications 

 through its substance, constituting Glissdn's capsule. 



Structure of the Liver. The liver is made up of a large number 

 of small bodies (the lobules}, rounded or ovoid in shape, measuring 



