DIGESTION OF FOOD. 34.5 



axillary and two sacral lymph hearts. The latter are, espe- 

 cially, easily seen, and there is no doubt that they are under 

 the control of the nervous system. 



In the mammals no such special helps for the propulsion of 

 lymph exist. 



There is little doubt that the blood-pressure is always higher 

 than the lymph-pressure, and when the blood-vessels are dilated 

 the fluid within the perivascular lymph-channels is likely com- 

 pressed ; muscular exercise must act on the lymph-channels as 

 on veins, both being provided with valves, though themselves 

 readily compressible; the inspiratory e£Forts, especially when 

 forcible, assist in two ways : by the compressing effect of the 

 respiratory muscles, and by the aspirating effect of the negative 

 pressure within the thorax, producing a similar aspirating 

 effect within the great veins, into which the large lymphatic 

 trunks empty. The latter are provided at this point with 

 valves, so that there is no back-flow; and, with the positive 

 pressure within the large lymphatic trunks (thoracic duct, etc.), 

 the physical conditions are favorable to the outflow of lyinpb 

 or chyle. 



Our knowledge of the nature of the passage of the chyle 

 from the intestines into the blood is now clearer than it was till 

 recently, though still inojmplete. 



The exact structure of a villus is to be carefully considered. 

 If we assume that the muscular cells in its structure have a 

 rhythmically contractile function, the blind terminal portion 

 of the lacteal inclosed within the villus must, after being 

 emptied, act as a suction-pump to some extent; at all events, 

 the conditions as to pressure would be favorable to inflow of 

 any material, especially fluid without the lacteal. The great 

 difficulty hitherto was to understand how the fat found its way 

 through the villus into the blood, for, that most of it passes in 

 this direction there is little doubt. 



It is now known that leucocytes (amceboids, phagocytes) 

 migrate from within the villus outward, and may even reach 

 its surface, that they take up (eat) fat-particles from the epi- 

 thelium of the villus, and, independently themselves, carry 

 them inward, reach the central lacteal and break up, thus re- 

 leasing the fat. How the fat gets into the covering epithelium 

 is not yet so fully known — possibly by a similar inceptive pro- 

 cess ; nor is it ascertained what constructive or other chemical 

 processes it may perform; though it is not at all likely that 



