CAPILLARIES AND VEINS. 623 



the sap issues from the stump of a cut grape-vine, 

 as shown by the experiments of Mirbel, who 

 found it equal to the pressure of 34 inches of 

 mercury.* And such is the vis a tergo generated 

 by the capillary circulation of chyle and lymph 

 through the absorbents, that when the thoracic 

 duct of a dog is secured with a ligature below the 

 subclavian vein, it has been known to burst from 

 the pressure. In all such cases, the vis a tergo 

 is generated by virtue of an attraction between 

 fluids and their containing vessels; the former 

 being plus, and the latter minus. The organic 

 particles of blood are forced through the pores of 

 the systemic capillaries, and combined with the 

 several tissues, by virtue of the same attraction. 

 And it has been found that the worn out particles 

 of the solids are not taken up by the mouths only 

 of the lymphatics, but are absorbed by the pores 

 of their coats : and finally, that neither in plants 

 nor animals is any contraction of their capilla- 

 ries essential to their circulation. 



* It has been long known, that when a plant is put in the 

 ground, its roots will extend only in the direction where they can 

 obtain nourishment ; and that when placed in a dark room, with 

 a hole in the wall to admit light, it gradually extends or grows in 

 that direction. Nor is the latter fact more surprizing, than that 

 solar influence should be the cause of growth throughout the 

 vegetable creation. 



