UBINAEY SECEETION. 85 



sometimes they also are alkaline, but much more strongly 

 than the blood ; whilst some are characterized by the presence 

 of matters not to be found elsewhere, such as urea, casein, 

 butter, &c. 



160. It is probable that in all cases the secreted matter 

 exists in the blood already formed. It was thought, for 

 example, that the urea found in urine must be formed by 

 and in the kidneys, since it could not be detected by chemical 

 analysis in the blood; but if these organs be destroyed in a 

 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 

 perceptible 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 surrounded with much fat. 

 They are of a reddish-brown colour, and in shape resemble a 

 kidney-bean (Fig. 62). Their substance (Fig. 61) is com- 

 posed essentially of secreting tubes of extreme tenuity, and of 

 great length, which in mammals are turned on themselves in 

 every direction towards their free extremity, 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, so as to form a certain number of pyramidal fasciculi 

 (b), whose summit is partially enclosed by the membranous 



