UBINABY SECBETION. 81 



living animal, or removed, urea will, after a certain time, be 

 formed in the blood, thus clearly proving that the kidneys do 

 not form it. 



161. Nature of the Secreted Liquids. There is no per- 

 ceptible relation between the nature of the fluid and of the 

 gland secreting it ; and secretions, as pus, for example, are 

 formed by structures where no such secretion previously 

 existed ; they alter also without any visible change in the 

 structure of the gland. 



Nothing positive is known as to the nature of the secreting 

 function, but it is certain that the action of the nervous sys- 

 tem has a great influence over it. When the nerves of the 

 stomach have been divided in a living animal, the secretion 

 of the gastric juice ceases ; and M. Bernard has shown, that 

 when a certain portion of the spinal marrow is irritated, an 

 unusually abundant secretion of sugar takes place in the 

 liver, which sugar then appears in the urine. 



This fact is remarkable, viewed in connexion with the 

 disease called diabetes. 



Urinary Secretion. 



162. This function has its seat in the kidneys, two 

 large glands situated in the abdomen, on either side of the 

 vertebral column, and generally sur- 

 rounded with much fat. They are of a 

 reddish-brown colour, and in shape re- 

 semble a kidney-bean (Fig. 53). Their 

 substance (Fig. 52) is composed essen- 

 tially of secreting tubes of extreme 

 tenuity, and of great length, which in 

 mammals are turned on themselves in 

 every direction towards their free ex- 

 tremity, where they swell into the form 

 of an ampulla (a), and which afterwards 

 proceed in a straight line towards the 

 middle of the inner edge of the gland, 



P . ~ v Fig. o3. Lnnary 



so as to form a certain number ot pyra- Apparatus.* 



midal fasciculi (b), whose summit is 

 partially enclosed by the membranous cavity called calyx (c), 

 and whose base, directed outwards, is, as it were, encased in 

 the cortical substance of the kidney, formed out of the mass 



* a, the kidneys ; b, the ureter ; c, the bladder ; d, canal of the urethra. 

 G 



