STuUCTUEE OF GLANDS. 24 j 



lation, is most active where vessels most abound, and accord 

 ingly most copious from the surface of the lungs. It has been 

 estimated that, under certain circumstances, the human body 

 loses, by exhalation, five-eighths of the whole weight of sub 

 stances taken into it. 



[ 414. SECRETION is a more complicated process than ex- 

 halation. It is not a mere mechanical operation, but is ac- 

 complished by means of organs, called glands ; which elaborate 

 peculiar juices, such as the sweat, the tears, the milk, the saliva, 

 the bile, the urine, &c. 



[415. At first glance there would seem to be nothing in 

 common between the organs which secrete the tears and that 

 which produces the bile, or between the kidneys and the 

 salivary glands. Still they all have the same elementary 

 structure. Every gland is composed of minute vesicles, or 

 extremely thin membranous sacs, generally too small to be 

 discerned by the naked eye, but easily distinguished by the 

 microscope. Sometimes these vesicles are single, and open 

 separately at the surface ; they are then called crypts or fol- 

 licles, but more frequently they unite to form clusters opening 

 into a common canal, which itself unites with the canals of 

 similar clusters to form trunks of various sizes, such as are 

 found in the salivary glands (figs. 257 and 2/7), in the mam- 

 mse, or in the liver (figs. 265, 267), which is a very large 

 gland receiving a great quantity of blood from the veins of 

 the alimentary canal 



[ 416. Sometimes the canals of the little clusters do not 

 unite, but open separately upon the surface of the body or 

 into its cavities, as in the intestinal glands or those from which 

 the perspiration issues (fig. 250, e). Occasionally the canals 

 themselves combine into bundles composed of a multitude of 

 parallel tubes, as we find in the kidneys, figs. 260 262. T. W. I 



417. The operation of the glands is one of the most 

 mysterious phenomena of animal life. By virtue of the pe- 

 culiar properties with which they are endowed, they select 

 from the blood, which penetrates to their remotest ramifica- 

 tions, the elements of the special humours they are designed 

 to elaborate. Thus the liver extracts the elements of the bile ; 

 the salivary glands the elements of saliva ; the pancreas those 

 of the pancreatic juice ; and the sodoriferous glands those 

 of the sweat, 



