CHAP, i.] DIGESTION. 305 



lymph will be sucked in from the thoracic duct, while the increase 

 of pressure in the great veins during expiration is warded off from 

 the duct by the valve at its opening. In the third place, the flow 

 may possibly be increased by rhythmical contractions of the 

 muscular walls of the lymphatics themselves ; but this is doubtful, 

 since it is not clear whether the rhythmic variations which have 

 been observed in the lacteals of the mesentery of the guinea-pig are 

 active or simply passive, i.e. caused by the rhythmic peristaltic 

 action of the intestine, each contraction of the intestine filling the 

 lymph- channels more fully. Lastly, it is quite open for us to sup- 

 pose that just as osmosis may give rise to increased pressure on one 

 side of a diffusion septum, so the diffusion of substances from the in- 

 testines into the lacteals, or from the tissues into the lymphatics, 

 may be itself one of the causes of the flow of lymph. We have 

 at least, under all circumstances, one or other of these causes at 

 work promoting a continual flow from the lymphatic roots to the 

 great veins. We have no very satisfactory evidence that the flow of 

 lymph is in any way directly governed by the nervous system. We 

 cannot prove any direct connection between the nervous system 

 and absorption, though the phenomena of disease render such a 

 connection at least probable. 



That the nervous system does exert an influence on absorption 

 is shewn by the following experiment, though probably in this 

 case the influence is an indirect one carried out through the medi- 

 ation of the vascular system. Of two frogs placed under the in- 

 fluence of urari so as to do away with muscular movements and 

 the action of the lymph-hearts, the brain and spinal cord are de- 

 stroyed in the one but in the other are left intact. Both animals 

 are suspended by the lower jaw ; chloride of sodium solution (75 

 per cent.) is poured into the dorsal lymphatic sacs of both ; and in 

 both the aorta is cut across. In the one where the nervous system 

 is intact, absorption from the lymphatic sac takes place copiously 

 and the heart pumps out large quantities of fluid by the aorta, 

 In the other, absorption does not occur; the heart, though beating, 

 remains empty, and the skin becomes dry. The experiment 

 probably shews the influence of the nervous system in maintaining 

 the tonicity of the blood-vessels and keeping up the connection of 

 the heart with the peripheral vessels, rather than any direct con- 

 nection between absorption proper and the nervous system. 

 When the nervous system is destroyed, dilation of the splanchnic 

 vascular area causes all the blood to remain stagnant in the portal 

 vessels, and probably these as well as other veins are rendered 

 unusually lax, so that the blood is largely retained in the venous 

 system, and very little reaches the heart; and with the enfeebled 

 circulation the absorption from the lymphatic sac is slight. So 

 long as the nervous system is still intact this stagnation does not 

 occur, the blood reaches the heart as usual, and with the more 

 vigorous circulation absorption from the lymphatic sac goes on 



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