logical defenses of the thymectomized animal 

 do not function properly when a thymectomy 

 is performed during the first few days of 

 postnatal life. 



Miller and others have postulated that the 

 thymus represents the primary source of im- 

 munologically active lymphoid cells in the 

 young mammal. Mitotic activity in the fetal 

 and early postnatal thymus is very high. It 

 is presumed that many of the lymphoid cells 

 produced are soon despatched from the 

 thymus to the other lymphoid stations of the 

 body— particularly to the lymph nodes and 

 spleen — where multiplication continues. The 

 importance of the primary source, accord- 

 ingly, diminishes with age. Eventually, there- 

 fore, the thymus tends to atrophy. The actual 

 production of antibody proteins occurs 

 mainly in the lymph nodes, particularly in 

 masses of special cells, called plasma cells. 

 These are derived from the primary lym- 



The Circulatory System - 337 



phoid cells. In any event, the masses of 

 plasma cells become exceptionally conspicu- 

 ous in lymph nodes when these are located 

 near an infected area (p. 335). 



There are many unsolved problems in 

 immunology, however: Are there many kinds 

 of plasma cells, each responsible for the pro- 

 duction of a particular antibody protein; or 

 can one kind produce a variety of antibodies, 

 when stimulated by the presence of the cor- 

 responding antigens? How is the very precise 

 specificity of the antigen-antibody relation- 

 ship determined? Does the antigen provide 

 a template (p. 134) for the synthesis of a 

 corresponding antibody — and if so, how? 

 Considerable progress has been made recently 

 in analyzing the structure of the antibody 

 proteins in the gamma globulin (p. 505) frac- 

 tion of the plasma. Thus it seems likely that 

 some of these questions will, in the near 

 future, receive definitive answers. 



TEST QUESTIONS 



1. Why is a circulatory system essential in all 

 larger animals? What are the general func- 

 tions of the circulatory system? 



2. Specify two kinds of animals possessing di- 

 rect (hemolymph) circulations, and two kinds 

 of animals possessing indirect (blood-lymph) 

 circulations; explain the essential differences 

 between these types of circulation, and relate 

 these differences to the question of "effi- 

 ciency." 



3. Exchanges between the blood and the cells 

 of an organ cannot occur in the arteries and 

 veins, but only in the capillaries. Is this state- 

 ment true, and if so, why (at least two rea- 

 sons)? 



4. Differentiate between erythrocytes, leuco- 

 cytes, and thrombocytes on the basis of: (a) 

 size and form; (b) abundance in the blood; 

 (c) general functions; (d) origin. 



5. Differentiate between diapedesis and phago- 

 cytosis; explain the importance of these proc- 

 esses in relation to the body's defenses against 

 bacterial infection. 



6. What other defense mechanisms are local- 

 ized: (a) in the plasma; and (b) in the lymph 

 glands? 



7. Without vitamin K the body is unable to 



synthesize prothrombase. Predict the effects 

 of a diet that is very low in K, explaining 

 your answer fully. 



8. Propose three different methods by which you 

 might prevent a sample of blood from clot- 

 ting while you were withdrawing it into a 

 test tube. Carefully explain the basis of each 

 method in relation to the clotting mecha- 

 nisms. 



9. Differentiate between arteries, veins, and 

 capillaries as to: (a) structure; (b) function; 

 (c) the relative pressure of the conducted 

 blood. 



10. Explain the physiological basis for the adage 

 "a man is as young as his arteries." stressing 

 the importance of the "elasticity factor." 



11. Make a labeled diagram to show the relations 

 of a capillary to the lymph and the surround- 

 ing tissue cells. 



12. Assuming that the foregoing tissue is a mus- 

 cle, explain how and why: 



a. glucose passes from the blood into the 

 cells 



b. oxygen passes from the blood into the 

 cells 



c. carbon dioxide passes from the cells 

 into the blood 



