381 LYMPH AND LYMPHATIC GLANDS [CH. XXIII. 



The lymph enters the gland by several afferent vessels, which 

 pierce the capsule and open into the lymph-path ; at the same time 

 they lay aside all their coats except the endothelial lining, which is 

 continuous with the lining of the lymph-path. The efferent vessels 

 begin in the medullary part of th gland, and are continuous with 

 the lymph-path here as the afferent vessels are with the cortical 

 portion. 



The efferent vessels leave the gland at the hilus, and either at 

 once, or very soon after, join together to form a single vessel. 



Blood-vessels which enter and leave the gland at the hilus are 

 freely distributed to the trabecular and lymphoid tissues. 



The Lymph Flow. 



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 propelling the chyle ; 

 in the small intestine of many animals the chyle has been seen 

 moving with intermittent propulsions that correspond with the peri- 

 staltic movements of the intestine. For the general propulsion of 

 the lymph and chyle, it is probable that, in addition to external 

 pressure, some of the force is 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 amphibia, 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 positions 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 pre- 

 vented 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 correspond- 

 ing 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 upofi 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 lymph-hearts. 



