ABSORPTION BY BLOODVESSELS. 293 



by valves, which prevent the retrograde passage of the lymph. 

 From each heart a single vein proceeds and conveys the lymph 

 directly into the venous system. In the frog, the inferior lym- 

 phatic heart, on each side, pours its lymph into a branch of 

 the ischiatic vein ; by the superior, the lymph is forced into a 

 branch of the jugular vein, which issues from its anterior sur- 

 face, and which becomes turgid each time that the sac contracts. 

 Blood is prevented from passing from the vein into the lym- 

 phatic heart by a valve at its orifice. 



The muscular coat of these hearts is of variable thickness ; 

 in some cases it can only be discovered by means of the micro- 

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

 fibres. The contractions of the hearts are rhythmical, occur- 

 ring 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 synchronous 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 certain 

 limited portion of the spinal cord. For Volkmann found that 

 so long as the portion of spinal cord corresponding to the third 

 vertebra of the frog was uninjured, the cervical pair of lym- 

 phatic 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 parts of the nervous centres 

 were uninjured, instantly arrested the heart's movements. The 

 posterior or ischiatic pair of lymph-hearts were found to be 

 governed, in like manner, by the portion of spinal cord cor- 

 responding to the eighth vertebra. Division of the posterior 

 spinal roots did not arrest the movements ; but division of the 

 anterior roots caused them to cease at once. 



Absorption by Bloodvessels. 



The process thus named is that which has been commonly 

 called absorption by the veins; but the term here employed 

 seems preferable, since, though the materials absorbed are 

 commonly found in the veins, this is only because they are 

 carried into them with the circulating blood, after being ab- 

 sorbed by all the bloodvessels (but chiefly by the capillaries) 

 with which they were placed in contact. There is nothing in 

 the mode of absorption by bloodvessels, or in the structure of 

 veins, which can make the latter more active than arteries of 

 the same size, or so active as the capillaries, in the process. 



In the absorption by the lymphatics or lacteal vessels just 



25 



