284 ABSORPTION AND SANGUIFICATION. 



part ; the latter collect that which is transmitted from the anterior part 

 of the body and head, and empty their contents into the jugular vein. 

 Their pulsations are totally independent of the action of the heart, and 

 of the respiratory movements ; since they continue after the removal of 

 the former, and for an hour or two subsequently to the death and com- 

 plete dismemberment of the animal. They usually take place at the 

 rate of about sixty in the minute ; but they are by no means regular, 

 and are not synchronous on the two sides. 



500. In Birds, we find the Absorbent system existing in a more per- 

 fect form ; its diffused plexuses and convolutions being replaced by glands ; 

 in which the contained fluid is brought into closer proximity with the 

 blood, and in which it is subjected to the influence of assimilating cells. 

 These, however, are not very numerous ; being principally found on the 

 lymphatics of the upper extremities. The absorbents, in this class, 

 terminate principally by two thoracic ducts, one on each side, which 

 enter the jugular veins by several orifices. There are, however, two 

 other entrances, as in Reptiles, into the veins of the lower extremity ; 

 and these are connected with two large dilatations of the lymphatics, 

 which are evidently analogous to the lymphatic hearts of Reptiles, but 

 which have little or no power of spontaneous contraction. In Mam- 

 malia, the Absorbent system presents itself in its most developed and 

 concentrated state. The vessels possess firmer walls, and are more 

 copiously provided with valves, than in the classes beneath ; and the 

 glands are much more numerous, particularly upon the vessels that 

 receive or imbibe substances from without, as those of the digestive 

 cavity, the skin, and the lungs. The terminations of the absorbents in 

 the veins are usually restricted, as in Man, to the single point of en- 

 trance of the thoracic duct on either side ; but they are sometimes more 

 numerous ; and certain variations in the arrangement of the thoracic 

 ducts, which occasionally present themselves as irregularities in Man, 

 are the ordinary conditions of these parts in some of the lower Mam- 

 malia. 



501. With regard to the source of the matters absorbed by the 

 Lymphatics, it is difficult to speak with certainty. We shall presently 

 see that their contents bear a close resemblance to the fluid element of 

 the blood, or " liquor sanguinis," in a state of dilution ; and it is very 

 probable that they partly consist of the residual fluid, which, having 

 escaped from the blood-vessels into the tissues, has furnished the latter 

 with the materials of their nutrition, and is now to be returned to the 

 former. But they may include, also, those particles of the solid frame- 

 work, which have lost their vital powers, and which are, therefore, not 

 fit to be retained as components of the living system, but which have not 

 undergone a degree of decay which prevents them from serving, like 

 matter derived from the dead bodies of other animals, as a material for re- 

 construction, when it has been again subjected to the organizing process. 



502. It was formerly supposed (and the doctrine was particularly in- 

 culcated by the celebrated John Hunter) that the office of the Lymphatic 

 system is to take up and remove all the effete matter, that is to be cast 

 out of the body, being no longer fit for its nutrition. But for such a 

 supposition there is no adequate foundation. It seems absurd to ima- 



