EESPIRATION" 175 



longer share in the expansions and contractions of the lungs, 

 and imperfect ventilation of this part of the lung results. 



10. Secondary effects of the breathing movements. The 

 student will now be better able to- understand the part 

 taken by the breathing movements in facilitating the return 

 of blood and lymph to the heart. The enlargement of the 

 thorax during inspiration sucks blood and lymph in toward 

 the great veins by the same process that it sucks air into 

 the lungs. Especially in the case of the lymph flow is this 

 a most important factor. Moreover, in the lymphatics of the 

 lungs, situated as they are entirely within the thorax, the 

 movements of the lungs during respiration pump the lymph 

 onwards and are of special importance in this respect. Much 

 of the invigorating effect of muscular exercise, popularly 

 ascribed to better oxygenation of the blood and tissues, is 

 really attributable to the greatly improved lymph flow from 

 all organs which results from the deepened respiration in 

 muscular activity. 



11. The automatic respiratory center and its regulation by 

 the carbon dioxide of the blood. The muscles of the dia- 

 phragm and those of the ribs, like the biceps and other 

 muscles which act upon the skeleton, are stimulated to con- 

 traction by nervous impulses from the brain and spinal cord. 

 Every movement of respiration is called forth and regulated, 

 in accordance with the needs of the body at the time, by the 

 coordinated action of a number of nerve cells. Those which 

 are most intimately concerned with respiration are found in 

 different parts of the central nervous system, from the lower 

 portion of the brain to the end of the first half of the spinal 

 cord, inclusive ; and there is good reason for thinking that 

 a group of nerve cells, usually known as the respiratory center, 

 in the lower portion of the brain, send out stimuli to those 

 of the cord and through them excite the muscles to contract. 



The respiratory center, like the heart (see p. 159), is auto- 

 matic. This means that its nerve cells periodically (usually 



