in the groin and armpits; and along the sides 

 of the neck. 



The lymph nodes constitute a reserve line 

 of defense against bacterial infection. The 

 lymphoid tissues in the nodes (and in the 

 tonsils, spleen, and intestinal wall) give rise 

 to the lymphocytes (p. 322); and the nodes 

 also tend to "strain" the lymph as it passes 

 through. If bacteria from the site of an in- 

 fection manage to elude the locally mobilized 

 phagocytes, they are carried in the lymph 

 stream toward the more vital centers of the 

 body. But in passing through the lymph 

 glands, most if not all of these bacteria are 

 likely to be destroyed by the phagocytes in 

 the nodes. Accordingly, lymph glands are 

 frequently swollen during any infection; and 

 the position of the swollen glands provides 

 an index of the site of the infection: a 

 swelling of the nodes of the groin indicates 

 a leg infection; swellings in the armpit indi- 

 cate an infection in the arm. 



Edema. Any part of the body tends to swell 

 if lymph accumulates in the tissue spaces. 

 Such a condition, which is called edema, 

 may result from a variety of factors, such as: 

 (1) high blood pressure, as in certain kinds 

 of heart disease; (2) low plasma tonicity due 

 to an excessive loss of plasma proteins, as in 

 chronic kidney disease; (3) abnormally per- 

 meable capillaries, due to a severe local dam- 

 age of the tissues (for example, spraining an 

 ankle); or (4) obstruction of the lymphatics 

 draining the particular part. In edema, the 

 skin and other tissues become very puffy; 

 and if pressed, edematous tissue remains in- 

 dented much longer than is normal. 



The Spleen. The spleen is a large reddish- 

 brown organ (Fig. 17-16) occupying a dorsal 

 position in the upper part of the abdominal 

 cavity, where it comes in contact with the 

 dorsal surface of the stomach and the upper 

 extremity of the left kidney. Structurally the 

 spleen resembles a lymph node, except for 

 its size, which is somewhat larger than the 

 kidney. Also the spleen possesses considerable 

 muscle tissue in its capsule and internal 

 framework. This permits the organ to ex- 



The Circulatory System - 335 



pand and contract, increasing or decreasing 

 the quantity of blood contained within its 

 spongy structure. 



For many years the function of this vas- 

 cular organ remained obscure, but now it is 

 clear that the spleen serves mainly as a sort 

 of blood bank from which reserves of blood, 

 and particularly of red cells, may be mobi- 

 lized when needed in the circulation. In the 

 cat, for example, it is estimated that as much 

 as one quarter of the erythrocytes and one 

 sixth of the plasma may be present in the ex- 

 panded spleen; and most of these reserves 

 can be launched into circulation when the 

 spleen contracts completely, as it does dur- 

 ing heavy muscular activity, or after severe 

 hemorrhage. In addition, the spleen has 

 several minor functions. It cooperates with 

 the other lymphoid tissues of the body in the 

 production of lymphocytes and in the phago- 

 cytosis of effete erythrocytes. In the embryo, 

 moreover, the spleen takes part in the pro- 

 duction of new erythrocytes. 



Antibody Production; Role of the Thymus 

 Glands. It has long been accepted that 

 lymphoid cells contribute heavily to the pro- 

 duction of antibodies in the body. However, 

 no involvement of the thymus gland was sus- 

 pected until some recent thymectomy (sur- 

 gical removal of the thymus) experiments 

 were reported by Jacques F. A. P. Miller, an 

 Australian investigator, working at the Ches- 

 ter Beatty Research Institute in London. In 

 fact, the functional importance of the 

 thymus, a gland that lies subjacent to the 

 upper part of the breast bone, has long re- 

 mained a mystery, despite the fact that in 

 prepubertal mammals the thymus constitutes 

 a conspicuous mass of lymphoid tissue, ac- 

 counting for almost 1 percent of the body 

 weight. In postpubertal individuals, however, 

 the thymus tissue atrophies and finally it is 

 scarcely distinguishable from the surround- 

 ing adipose tissue. 



Miller used mice in his experiments and 

 other workers have obtained confirmatory 

 evidence with the same animal. But in addi- 

 tion, Sir Macfarland Burnet, another Aus- 



