GLANDS, GENERAL PHYSIOLOGY OF SECRETION. 251 



secretion is here applied to all the products of all the 

 glands. Before following out in detail, now, what the ex- 

 act process of secretion is, it is necessary to turn to the 

 anatomy of glands. 



1. Glands and Their Anatomy. Here additional am- 

 biguities are at once misleading. There are a number of 

 structures in the body designated as glands which are not 

 true glands in any sense of that term. They were called 

 glands by anatomists who were ignorant of their real 

 nature, and these names given to them in this way still 

 cling to them in spite of our knowledge that they are not 

 glands at all. Just as we call the primitive inhabitants of 

 America Indians without intending to express in any way 

 the misconception that they are residents of the East 

 Indies. Such entirely different structures are the pineal 

 gland of the brain, which instead of being a gland is 

 really but the stump of an optic nerve. Such structures, 

 too, are the lymphatic glands, which are aggregations of 

 white blood corpuscles, and have absolutely nothing to do 

 with secretion in any way. 



In addition to the structures just mentioned, which are 

 at the very first glance seen to belong to entirely different 

 tissues, we have several structures which, while they may 

 be the seat of chemical changes in the blood passing through 

 them, yet have no ducts, and do not pour out distinct secre- 

 tions. They are, therefore, not infrequently called "duct- 

 less" glands. Examples of such are the spleen, the thyroid 

 gland and the adrenal glands. Disregarding all these, then, 

 it may be said that a typical gland consists of a basement 

 membrane of connective tissue bearing a surface of secret- 

 ing cells on one side, and supplied with numerous blood 

 vessels on the other. In addition to these three main ele- 

 ments there are, of course, nerves which in some instances 

 have actually been traced by anatomists into the secreting 

 cells themselves. Finally in the interstices, as in all other 

 tissues, are the lymphatics. It will be seen from this that 



