48 THE BODY AT WORK 



called upon to do work develops a greater capacity for 

 work. 



When nutrition is not at its best, the growth of muscle may 

 be favoured by external pressure which squeezes lymph out of 

 its tissue-spaces, and therefore leads to increased exudation 

 from the blood. It is not improbable that in badly nourished 

 tissues the circulation of blood is somewhat torpid and the 

 lymph stagnant. A feeble circulation usually results in some 

 oedema. The muscles, or rather the connective tissue which 

 envelops and penetrates them, feels doughy, instead of being, 

 as it should be, firm and elastic. Under these conditions 

 massage is undoubtedly of service. Squeezing the muscles 

 displaces lymph, and, if the pressure is properly directed, drives 

 it along the lymphatic vessels. Fresh lymph exudes from the 

 capillary bloodvessels, and the muscle- fibres, surrounded with a 

 more abundant supply of nutriment, benefit, as, in a vigorous 

 person, they benefit from use. 



Lymph is an exudate from blood. Its composition therefore 

 depends upon that of blood-plasm, but it tends to differ from 

 it owing to the influence of two causes. In the first place, the 

 walls of the capillary bloodvessels restrict exudation. Red 

 blood-corpuscles cannot pass through them. Proteins which 

 are non-diffusible are, according to the circumstances of the 

 tissues, held back to a greater or to a less extent. The pseudo- 

 capillaries of the liver let them pass, as has already been said. 

 The capillaries of the limbs restrict their passage to such pro- 

 portions as, it may be supposed, are absolutely necessary for 

 the nutrition of the tissues. In the second place, tissues remove 

 food from lymph and add to it waste products. Hence the 

 lymph issuing from a limb, after full contact with the tissues, 

 contains less of the former and more of the latter less sugar, 

 for example, and rather more oxidized nitrogenous substances, 

 lecithin and other things termed collectively " extractives," 

 because they can be extracted from dried blood or lymph by 

 ether. The reaction of lymph is alkaline. After a time it 

 coagulates, but coagulation is slower, and the clot less firm 

 than in the case of blood. 



As the composition of lymph depends upon the source from 

 which, and the conditions under which, it has been obtained, 

 it is unnecessary to state the results of a chemical analysis. 



