CH. XXII.] 



THE FLOW OF LYMPH 



317 



the lymph-path here as the afferent vessels aro 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 trabe- 

 cular and lymphoid tissues. 



> 



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 pro- 

 pels 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 propel- 

 ling the chyle ; in the small in- 

 testine of a mouse, the chyle has 

 been seen moving with intermit- 

 tent 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 vis a tergo resulting 

 from external pressure, some of the force is derived from the con- 

 tractility 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. 



FIG. 306. A small portion of medullary substance 

 from a mesenteric gland of the ox. d, d, trabe- 

 culse ; a, part of a cord of lymphoid tissue from 

 which all but a few of the lymph-corpuscles 

 have been washed out to show its supporting 

 meshwork of retiform tissue and its capillary 

 blood-vessels (which have been injected, and 

 are dark in the figure); ft, b, lymph-path, of 

 which the retiform tissue is represented only 

 at c, c, x 300. (Kolliker.) 



