LIVER. 



155 



Mechanism of Urinary Secretion. As the kidney anatomically presents 

 an apparatus for nitration (the Malpighian bodies) and an apparatus for 

 secretion (the epithelial cells of the urinary tubules), it might be inferred that 

 the elimination of the constituents of the urine is accomplished by the two- 

 fold process of filtration and secretion; that the water and highly diffusible 

 inorganic salts simply pass by diffusion through the walls of the blood-vessels 

 of the glomerulus into the capsule of Miiller, while the urea and remaining 

 organic constituents are removed by true secretory action of the renal epi- 

 thelium. Modern experimentation supports this view of renal action. 



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



The filtration of urinary constituents from the glomerulus into Muller's 

 capsule depends largely upon the blood-pressure and the rapidity of blood 

 flow in the renal artery and glomerulus. Among the influences which in- 

 crease 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 activity 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 hypochondriac region, and is 

 retained in position by five ligaments, four of which are formed by duplica- 

 tures 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 



