LYMPHATIC HEARTS. 



369 



branch of the ischiatic vein ; by the superior, the lymph 



Fig. 96*. 



is forced into a branch of the 

 jugular vein, which issues from 

 its anterior surface, and which 

 becomes turgid each time that 

 the sac contracts. Blood is pre- 

 vented from passing from the 

 vein into the lymphatic heart by 

 a valve at its orifice. 



The muscular coat of these 

 hearts is of variable thickness ; 

 in some cases it can only be dis- 

 covered by means of the micro- 

 scope ; but in every case it is composed of transversely- 

 striated fibres. The contractions of the hearts are rhyth- 

 mical, occurring about sixty times in a minute, slowly, and, 

 in comparison with those of the blood-hearts, feebly. 

 The pulsations of the cervical pair are not always synchro- 

 nous with those of the pair in the ischiatic region, and 

 even the corresponding sacs of opposite sides are not 

 always synchronous in their action. 



Unlike the contractions of the blood-heart, those of the 

 lymph-heart appear to be directly dependent upon a cer- 

 tain limited portion of the spinal cord. For Volkniann 

 found that so long as the portion of spinal cord correspond- 

 ing to the third vertebra of the frog was uninjured, the 

 cervical pair of lymphatic hearts continued pulsating after 

 all the rest of the spinal cord and the brain was destroyed ; 

 while destruction of this portion, even though all other 



* Fig. 96. Lymphatic heart (9 lines long, 4 lines broad), of a large 

 species of serpent, the Python bivittatus; (after E. Weber). 4. The 

 external cellular coat. 5- The thick muscular coat. Four muscular 

 columns run across its cavity, which communicates with three lympha- 

 tics (i only one is seen here), with two veins (2, 2). 6. The smooth 

 lining membrane of the cavity. 7. A small appendage, or auricle, the 

 cavity of which is continuous with that of the rest of the organ. 



BB 



