358 HISTORY OF THE HUMAN BODY 



are intermuscular or sub-fascial, their locations being desig- 

 nated by their names, and in tailless amphibians there is found 

 an extensive series of sub-cutaneous lymph-sacs, some of great 

 extent. 



These various spaces are in communication with one another 

 and usually communicate with the venous system in four 

 places: anteriorly with the two jugulars at or near their union 

 with the subclavians; and posteriorly with either the caudal 

 vein or the posterior cardinals near the entrance of the iliacs. 

 It is to be noted that these four points at which the lymphatic 

 and circulatory systems communicate are associated with the 

 four limbs, and although the number of these points of com- 

 munication is decreased in the higher vertebrates through a 

 suppression in the adult of certain of these, there are no new 

 ones formed, and the lymphatic system, even in its most spe- 

 cialized form, still follows in this particular the lines laid down 

 for it from the first. 



In fishes the lymphatic vessels, near their entrance into the 

 veins, enlarge into thin-walled sinuses, organs which in tail- 

 less amphibians develop into pulsating sacs with muscular 

 walls, the so-called lymph-hearts, the action of which furthers 

 the flow of the lymph (Fig. 163). Each lymph -heart pos- 

 sesses a single venous ostium, by which the sac communicates 

 ^wlth the vein,~and several lymphatic ostiq, through which the 

 sac receives the fluid from as many lymphatic vessels. The 

 forrneY^)peningjs^ equipped with two semi-lunar valves to pre- 

 vent filling the sac with blood during its expansion, but the 

 lymphatic ostia are without special valves. The tailed am- 

 phibians seem to lack the anterior pair of lymph-hearts, but 

 here, in addition to the posterior pair, a series of small pul- 

 sating sacs occur along the lateral line. 



Progress in the history of the lymphatic system among the 

 higher vertebrates is shown along two directions : first, in the 

 formation of more and more vessels with walls of their own, 

 the definite lymphatics, and, second, by the reduction of the 

 lyrriph-hearts. Thus in" reptiles only the posterior lymph- 

 hearts persist in the adult, and the same appear in birds dur- 



