3l6 LYMPH AND LYMPHATIC GLANDS. [CH. xxn. 



The Lymph Plow. 



The flow of the lymph towards the point of its discharge into 

 the veins is brought about by several agencies. With the help of 

 the valvular mechanism all occasional pressure on the exterior 

 of the lymphatic and lacteal vessels propels the lymph onward ; 

 thus muscular and other external pressure accelerates the flow of 

 the lymph as it does that of the blood in the veins. The action 

 of the muscular fibres of the small intestine, and the layer of 

 unstriped muscle present in each intestinal villus, assist in pro- 

 pelling the chyle ; for, in the small intestine of a mouse, the 

 chyle has been seen moving with intermittent propulsions that 

 correspond with the peristaltic movements of the intestine. But, 

 for the general propulsion of the lymph and chyle, it is probable 

 that, together with the v is a tergo resulting from external pressure, 

 some of the force may be derived from the contractility of the 

 vessel's own walls. The respiratory movements, also, favour 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. 

 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 vessel proceeds, and conveys the lymph directly 

 into the venous system. Blood is prevented from passing 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 rhythmical, 

 occurring about sixty times in a minute. 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 hearts' movements. 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 movements ; but division of the anterior roots caused 

 them to cease at once. 



Innerratwn of the Thoracic Duct. By determining the rate of outflow of 

 a fluid at constant pressure passing through the thoracic duct Camus and 



