Chap. XII] GENERAL CIRCULATION OF BLOOD 223 



with the pecuHarities of the individual. Age has a marked influ- 

 ence. At birth the pulse rate is about 130 per minute ; at three 

 years, 100 ; in adult life, 72 ; in old age, 65. It is somewhat more 

 rapid in women than in men and is lowered during sleep. Idiosyn- 

 crasies are frequently met with. A person in perfect health may 

 have a much higher or a much lower rate than 72. The relative 

 frequency of the pulse and respirations is about four heart -beats 

 to one respiration. 



As a rule, the rapidity of the heart's action is in inverse ratio 

 to its force. An infrequent pulse, within physiological limits, 

 is usually a strong one, and a frequent pulse comparatively 

 feeble. The same is true in disturbance of the heart's action 

 in disease, the pulse in fever or debilitating affections becoming 

 weaker as it grows more rapid. 



LYMPH 



Formation of lymph. — The lymph is derived from the blood 

 in the capillaries, but the exact process is still an open question. 

 It is considered probable that it is partly a process of transudation 

 which depends on the permeable nature of the walls of the capil- 

 laries, and partly the result of a secretory process on the part of 

 the endothelial cells lining the capillaries. The transudation 

 theory is supported by the fact that the blood in the capillaries 

 is under greater pressure than in the arteries or veins. The 

 secretory process is supported by the chemical differences between 

 the blood and the lymph. 



Factors controlling the flow of lymph. — The onward progress 

 of the lymph from the tissues to the veins is maintained chiefly 

 by three things. 



(1) Differences in pressure. — The lymph in the tissue spaces 

 is under greater pressure than the lymph in the lymph capillaries, 

 and the pressure in the large lymphatics near the ducts is much less 

 than in the smaller vessels. Consequently we may consider that 

 the lymphatics form a system of vessels leading from a region 

 of higher pressure, viz. the lymph spaces of the tissues, to a 

 region of lower pressure, viz. the interior of the large veins of 

 the neck. 



(2) Muscular movements and valves. — The muscular move- 

 ments of the body compress the hinphatics and force the lymph 



