274 THE BLOOD AND THE LYMPH 



present in very variable quantities but usually in numbers not very 

 different from those of blood namely eight to twelve thousand per cubic 

 millimeter. The number of the thrombocytes has not been determined. 



THE QUANTITY OF THE LYMPH produced and poured out of the two 

 ducts into the subclavian veins is commonly stated to be every day one- 

 thirteenth the body's weight. This is the same amount as that of the 

 blood present on the average in the body, or nearly five liters. Like 

 everything- else lymphatic, this too is very variable, for it depends on 

 the general blood-pressure, on the composition of the blood, on the 

 capillary permeability, on the nature and degree of the metabolic activity 

 in the tissues. Especially, as Hough has emphasized, does it depend 

 on the activity of the skeletal muscles, on the amount of physical exercise. 

 Lymph moves in a sort of circulation of its own. It is passed out of the 

 direct blood-circulation, soaks through the tissues, and afterward enters 

 again its system of "closed" lymph-vessels on its way again into the set 

 of "closed" blood-tubes. Because of this soaking through the tissue- 

 protoplasm and of the inexhaustible supply afforded by the circulating 

 blood, anything which increases the metabolism of much tissue on the 

 one hand, or which forces mechanically the onward lymph-flow on the 

 other, will increase the quantity of lymph passed out of the thoracic 

 ducts. The muscles constitute half the mass of the body and by their 

 activity increase in both of the ways last mentioned the flow of lymph. 

 Their heightened metabolism enlarges the demand for the food- and 

 oxygea-supplying lymph and their contractions compress the lymphatic 

 vessels and so crowd the lymph onward while the valves prevent its 

 movement in the backward direction. Measurements have proved 

 that an active muscle exudes five or six times as much lymph as the same 

 muscle in a passive state. Active muscles then suck in as well as vigor- 

 ously press out a large amount of lymph, and the draft thus produced 

 cannot fail to increase the lymph-flow all over the body. This means 

 in turn not only a better cleansing of the tissues of their excreta, but a 

 largely increased food-supply for them, more oxygen, growth, vigor. 

 This in part is doubtless the reason that physical exercise plays so large 

 a part in sustaining or increasing the good health of all animals and of 

 none no more than man, especially since his organism has been evolved 

 on much more bodily exercise than the average man and woman of 

 today provides it. 



THE FUNCTIONS OF THE LYMPH scarcely need extended systematic 

 description, for some of them have come out in the foregoing pages, 

 namely the conveyance of food and of oxygen inward from the capillaries 

 to the cells and the excretion into the capillary-blood of the tissues' waste. 

 It supplies, then, the material from which the body-cells may feed them- 

 selves. There are, however, at least four other lymphatic functions 

 which require mention: absorption of fat from the gut; the lubrication of 

 the great serous surfaces; the maintenance in part of the blood's composi- 

 tion; and the filling of the cerebral ventricles. 



In the movements of the viscera on each other, the heart between the 



