100 



OF THE SECRETIONS. 



complished by means of peculiar organs, called glands ; 

 which elaborate peculiar juices, such as the sweat, the 

 tears, the milk, the saliva, the bile, the urine, &c. 



266. 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 element- 

 ary structure. Every gland is composed of minute vesi- 

 cles, or extremely thin membranous sacs, generally too 

 small to be discerned by the naked eye, but easily distin- 

 guished by the microscope. Sometimes these vesicles are 



single and open sepa- 

 rately at the surface ; 

 they are then called 

 crypts or follicles, 

 but more frequently 

 they unite to form 

 Fig. 93. 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 (Fig. 92), 



in the mamma?, or in the liver, which is 



merely a very large gland receiving a 



large quantity of blood from the veins 



of the alimentary canal. Fig. 94. 



267. 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. 94, g). Occasionally, the 

 canals themselves combine into bundles composed of a mul- 

 titude of parallel tubes, as we find in the kidneys. 



268. The operation of the glands is one of the most ex- 

 traordinary and mysterious of the whole organization. By 

 virtue of the peculiar properties with which they are en- 



