ABSORPTION. 417 



small intestine, and probably the layer of unstriped muscle present in 

 each intestinal villus, seem to assist in propelling the chyle : for, in the 

 small intestine of a mouse, the chyle has been seen moving with inter- 

 mittent propulsions that appeared to correspond with the peristaltic 

 movements of the intestine. But for the general propulsion of the 

 lymph and chyle, it is probable that, together with (3) the vis a tergo 

 resulting from absorption (as in the ascent of sap in a tree), and from 

 external pressure, some of the force may be derived (4) from the con- 

 tractility of the vessel's own walls. The respiratory movements, also, 

 (5) favor the current of lymph through the thoracic duct as they do the 

 current of blood in the thoracic veins. 



Lymph-Hearts. In reptiles and some birds, an important auxiliary to the 

 movement of the lymph and chyle is supplied in certain muscular sacs, named 

 lymph-hearts, and it has been shown that the caudal heart of the eel is a 

 lymph-heart also. The number and position of these organs vary. In frogs 

 and toads there are usually four, two anterior and two posterior ; in the frog, 

 the posterior lymph -heart on each side is situated in the ischiatic region, just 

 beneath the skin ; the anterior lies deeper, just over the transverse process of 

 the third vertebra. Into each of these cavities several lymphatics open, the 

 orifices of the vessels being guarded 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 lymphatic 

 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 surface, 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 microscope ; but in every case it is 

 composed of striped fibres. The contractions of the hearts are rhythmical, 

 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 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 lymphatic 

 hearts continued pulsating after all the rest of the spinal cord and the brain 

 were destroyed ; while destruction of this portion, even though all other parts 

 of the nervous centres were uninjured, instantly arrested the heart's move- 

 ments. The posterior, or ischiatic, pair of lymph- hearts were found to be 

 governed, in like manner, by the portion of spinal cord corresponding to the 

 eighth vertebra. Division of the posterior spinal roots did not arrest the move- 

 ments ; but division of the anterior roots caused them to cease at once. 



Lymphatic Glands. Lymphatic glands are small round or oval 

 compact bodies varying in size from a hemp-seed to a bean, interposed 



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