314 THE TISSUES AND MECHANISMS OF DIGESTION. 



once say, the greater part of the fat passes, are similarly parts of the general 

 lymphatic system, being in fact the lymphatic vessels of the alimentary canal, 

 and especially of the small intestine. The only reason for the special name 

 of lacteals is that, unlike the lymphatic vessels of other parts of the body, 

 the lymphatics of the intestine contain at times a fluid of a milky white 

 appearance. Hence for the better understanding of absorption by the lac- 

 teals it will be desirable to study at some length the whole subject of the 

 lymphatic system. 



The lymphatic vessels may be said to begin in minute passages, possess- 

 ing special characters, known as lymph-capillaries. Broadly speaking, these 

 lymph-capillaries are found, in the mammal, in all parts of the body in 

 which connective tissue is found ; and they have special connections with 

 those minute spaces in connective tissue which we have already more than 

 once spoken of as lymph-spaces. 



These lymph-capillaries, which, as we shall see, are frequently arranged 

 in plexuses, are continuous with other passages also minute but of a differ- 

 ent and more regular structure, the lymphatic vessels proper, which are 

 gathered into larger and larger vessels, all running like the bloodvessels in 

 a bed of connective tissue, until at last all the lymphatic vessels of the body 

 join either the great thoracic duct which opens by a valvular orifice into 

 the venous system at the junction of the left jugular and subclavian veins, 

 or the small right lymphatic trunk which similarly opens into the junction 

 of the right jugular and subclavian veins. Serous cavities may be regarded 

 as large lymph-spaces connected with lymphatic vessels. 



THE NATURE AND MOVEMENTS OF LYMPH (INCLUDING CHYLE). 



247. We are led to regard the multitudinous spaces, both small and 

 great, of connective tissue all over the body, including among these the 

 " serous cavities," as forming the beginning or roots of the lymphatic sys- 

 tem. Into these spaces certain parts of the plasma of the blood transude 

 and so become lymph (to what extent the epithelioid lining of the large 

 serous cavities plays in regulating the transudation of serous fluid, i. e., of 

 lymph, into those cavities we do notknowj ; from these spaces the lymph is 

 continually flowing through the lymph-capillaries into the lymphatic vessels, 

 and so by the thoracic duct and right lymphatic trunk back into the blood 

 system. 



The amount of lymph occupying the lymph-spaces, lymph-capillaries, 

 and minute lymphatic vessels of any region varies from time to time accord- 

 ing to circumstances. A hand, for instance, which has been kept hanging 

 down for some time becomes swollen and the skin tense ; if it be raised the 

 swelling lessens and the skin becomes loose ; and a similar temporary swell- 

 ing of the skin of the limbs, and of the skin generally, is frequently the 

 result of active exercise. Such a swelling is partly due to the bloodvessels 

 being dilated, or to the return flow along the veins being retarded, so that 

 the blood capillaries become distended with blood, but is much more largely 

 owing to the lymph-spaces and lymphatic vessels of the skin and under- 

 lying structures being unusually filled with lymph. On the other hand 

 the skin may become shrivelled and dry from a deficiency of lymph in 

 the lymph spaces and vessels. Under even normal circumstances the 

 quantity of lymph in the tissues may vary considerably, and under abnor- 

 mal circumstances a very large amount of lymph may greatly distend 

 the spaces of the connective tissue of the skin and other structures, giving 

 rise to oedema or dropsy. Obviously there are agencies at work in the 

 body by which the appearance of lymph in the spaces or its removal 



