124 DISTINCTION BETWEEN SECRETION AND EXCRETION. 



examined microscopically and chemically. In every instance under the microscope 

 they were found to be composed of small granules and delicate acicular crystals. 



All the characteristic chemical tests gave unequivocal evidence that these granules 

 were composed of uric acid and ammonia. A careful microscopical examination 

 also showed that they were the urate of ammonia, which is the most abundant con- 

 stituent of the urine of serpents. 



From the results of this experiment we may draw the following conclusions : — 



1. The kidneys are excreting and not secreting organs. The circulatory appa- 

 ratus not only carries nutriment to the different organs and tissues, but also removes 

 from them the products of their disintegration and metamorphosis of no further use 

 in the animal economy. 



The amount and character of an excretion depends entirely upon the amount 

 and character of the excrementitious materials existing in the blood. 



A secretion does not exist in the blood. "We do not find the gastric juice or the 

 salivary fluid existing in the blood before they are elaborated by special organs. 



2. When the kidneys are amputated, other membranes and organs assume their 

 office of depurating the blood. In like manner, if the function of the skin be 

 checked it will be assumed by the kidneys. The act, then, of separating certain 

 materials from the blood can be transferred from one excretory organ to another. 

 This, however, is not true of secretions. Each secretion must have a special set of 

 cells, which alone can produce the peculiar material. 



"We never find one organ elaborating the secretion of another distinct organ. 

 This is a general law. The salivary gland never secretes gastric juice, nor the 

 mammillary gland, bile. The stomach of a Rattlesnake never secretes the deadly 

 fluid of its poison gland. Thus, in two essential respects, a secretion differs from 

 an excretion. 



3. It is probable that in the lower animals which are without kidneys, the office 

 of the latter is carried on by the mucous membrane of the stomach and intestinal 

 canal. 



We will next consider the relative size of the kidneys in the four great classes of 

 vertebrate animals. 



