THE FLUIDS OF THE BODY 45 



on the lymphatic vessels. As they are provided with valves, 

 squeezing them converts them into pumps. The fluid which 

 they contain is bound to go forwards. Additional fluid is 

 squeezed into them from the tissue-spaces. To a large extent, 

 therefore, the outflow of lymph from contracting muscles is 

 to be explained as the result of the pressure which the swelling 

 muscles exert upon the lymphatic vessels within their sheaths. 

 But there is another factor which must not be overlooked, 

 although it cannot readily be estimated. When a muscle 

 is actively contracting its bloodvessels dilate. There is a 

 greater exudation of lymph; and reabsorption by blood is 

 not equal to the exudation. The surplus leaves the limb 

 by the lymphatic vessels. A gland is never at rest. In the 

 intervals between the ejection of its secretion its cells are 

 preparing materials for the next outflow. Lymph is always 

 flowing from a gland ; its amount increases as the activity of 

 the gland increases. More lymph leaves the blood when the 

 gland is exceptionally active than when it is relatively quiet. 

 Some of it is not reabsorbed into the blood. A certain pro- 

 portion of the waste products of the active gland are hurried 

 away by the overflow system in the direction of the thoracic 

 duct. 



Lymph is the reservoir of nutriment upon which every cell 

 in the body draws. It is improbable that in health and under 

 normal conditions the activity of any organ is ever restricted 

 for want of sufficient food. As food is removed from lymph, 

 it is instantly replaced by fresh food from the blood. There 

 is some evidence not very clear that the removal of waste 

 products offers greater difficulty than the renewal of supplies 

 of food. When the activity of muscles has been excessively 

 prolonged they ache. It has been supposed that their un- 

 willingness to do more work is due, not to the exhaustion of 

 the food which they use up when contracting, but to the 

 inadequacy of the lymph and blood to carry off all refuse. 

 This, at least, is the explanation of fatigue which is usually 

 offered, although it is difficult to understand why the arrange- 

 ments for removing waste products which have worked to 

 perfection for eight hours should during the ninth hour become 

 rapidly ineffective. 



If a frog's muscle, cut out of the body, has been made to 



