510 LYMPH-HEARTS. [BOOK n. 



heart contracts the contents are driven into the vein, the lym- 

 phatic pores being closed by the approximation of the contracting 

 muscular fibres ; when the heart dilates, the fluid in the vein is 

 prevented from returning by the valves at its mouth, while the 

 lymph enters readily from the surrounding space through the now 

 open pores. In the frog regular lymphatic vessels are scanty ; 

 hence these lymph-hearts become of considerable importance in 

 promoting the flow of lymph. The lymph-hearts of reptilia are 

 similar in structure and function. In the frog, in which they 

 have been chiefly studied, the action of the lymph-hearts 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 is not an absolute one. Like the heart of 

 the blood-system, 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 pro- 

 cesses of absorption, are in this animal directly dependent on 

 the nervous system. 



