306 LYMPH HEARTS. [BOOK n. 



rapidly. As the blood is pumped away its place is renewed by the 

 lymph, supplied by the fluid in the sac, and thus the heart may be 

 made for a long time to pump away the fluid poured into the sac. 



Lymph hearts. In frogs and some other animals the centri- 

 petal flow of lymph from the limbs is assisted by rhythmically 

 pulsating muscular lymph hearts, which present many curious 

 analogies with the blood-heart. In the frog, in which they have 

 been chiefly studied, their action as we have already stated (p. 108) 

 is in a measure dependent on the spinal cord. The posterior lymph 

 hearts belonging to the hind limbs are connected by means of the 

 delicate tenth pair of spinal nerves, with a region of the cord 

 opposite the sixth or seventh vertebra, in such a way that section 

 of the nerve or destruction of the particular region of the cord 

 suspends or destroys their activity. The anterior pair are similarly 

 connected with a region of the spinal cord opposite the third 

 vertebra. Each pair therefore seems to have a 'centre' in the spinal 

 cord; but it is probable, though observers are not wholly agreed, 

 that the hearts, after destruction of their spinal centre, ultimately 

 resume their rhythmic beats, so that the dependence of their 

 activity on the spinal centre, like the similar dependence of the 

 blood heart on the ganglia of the sinus venosus, is not an absolute 

 one. Like the blood heart, the lymph hearts may be inhibited, 

 and that in a reflex manner, the inhibition centre being moreover 

 in the medulla oblongata. If a frog be carefully observed, the 

 activity of the lymph hearts will be found to vary largely, and these 

 variations appear to be in part due to nervous influences ; so that 

 in this way the movement of lymph, and hence the processes of 

 absorption, are in this animal directly dependent on the nervous 

 system. 



The course taken by the several products of digestion. 



Digestion being, broadly speaking, the conversion of non- 

 diffusible proteids and starch into highly diffusible peptone and 

 sugar, and the emulsifying, or division into minute particles, of 

 various fats, it is natural to suppose that the diffusible peptone 

 and sugar pass by osmosis into the portal vessels and so directly 

 into the blood, and that the emulsified fats pass into the lacteals 

 and so indirectly into the blood. That a large part of the fat 

 which enters the body from the intestine does pass through the 

 lacteals, there can be no doubt ; and there can be but little doubt 

 that a considerable quantity of peptone and sugar does pass into 

 the portal blood. But the question as to how far the fat in its 

 difficult passage into the lacteal is accompanied by soluble peptone, 



