CIRCULATION OF BLOOD AND OF LYMPH 



169 



Duct Sec re in 



times called ductless glands, which do not seem to belong to any 

 of the general systems. Since one type of these is associated 

 with the lymph system, all of them will be mentioned here. 



A gland (Fig. 95) is a collection of cells, the protoplasm 

 of which takes from the blood more fluid than is used in the 

 cell itself, and changes it chem- 

 ically, after which the extra 

 amount is secreted, or shed into 

 some cavity, (as the pericardial 

 cavity for example), or into some 

 narrow passage or duct (as for 

 instance, those of the salivary 

 glands). There are, however, 

 some glands in the body which 

 do not empty their secretions 

 into any duct or cavity; they are 

 plentifully supplied with blood 

 vessels, and so have much ma- 

 terial at hand from which to 

 make quantities of a special kind 

 of secretion. Whatever fluid they 

 make is poured directly into the 

 blood vessels and the substances they furnish are called 

 hormones. 



Lymph Glands. The first of these ductless organs are the 

 so-called lymph glands. These are small, more or less round 

 masses of tissue, scattered along the course of the lymph 

 vessels; Fig. 93. They are found in many parts of the body, 

 particularly at such places as the shoulder and hip joints. 

 At present practically nothing is known of their functions, 

 although it is believed that they are the Geat of white blood 

 corpuscle formation. 



The Spleen. The spleen is located near the lower wall of 

 the stomach and is supported there by folds of the mesentery; 

 Fig. 78, page 146. So plentiful is its blood supply that when 



Artery Vein 



OF 



FIG. 95. DIAGRAM 



SIMPLE GLAND 



Showing the secreting cells and the 

 blood vessels supplying them. 



