304 HANDBOOK OF PHYSIOLOGY. 



external pressure accelerates the flow of the lymph as it does that of the 

 blood in the veins. The actions of (2) the muscular fibres of the small 

 intestine, and probably the layer of unstriped muscle present in each in- 

 testinal villus, seem to assist in propelling the chyle: for, in the small 

 intestine of a mouse, the chyle has been seen moving with intermittent 

 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 visa/ tcrgo 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 contractility of the ves- 

 seFs 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 (Fig. 220), and it has been shown that the 



FIG. 220. Lymphatic heart (9 lines long, 4 lines broad) of a large species of serpent, the Python 

 bivittatus. 4. The external cellular coat. 5. The thick muscular coat. Four muscular columns 

 run across its cavity, which communicates with three lymphatics (1 only one is seen here), and 

 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 (after E. Weber). 



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 an- 

 terior 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 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 discovered by means of the microscope; but in every 

 case it is composed of striped fibres. The contractions of the hearts are 



