LYMPHATIC SYSTEM. 195 



the fluid may contract peristaltically, and press the fluid for- 

 wards from one pair of valves to another. 



The lymphatic system is very full of these valves, much more so 

 than the venous, and the reason of this difference seems to be, 

 that the blood in the venous system is strongly pressed forwards 

 by the vis a tergo from the action of the heart and arteries, and 

 therefore its course is less liable to be interrupted by any acci- 

 dental pressure. But the motion of the absorbed fluid in these 

 vessels having no such force, but only that of the attraction at the 

 orifice, and the peristaltic contraction of the coats, might easily 

 be overcome by any lateral compression, were it not for the 

 valves, which seem to be given to prevent the retrocession of 

 the lymph being considerable, and to make any lateral pres- 

 sure, instead of preventing, rather promote its passage to the 

 heart ; and as the lymphatic vessels in the human subject not 

 only accompany the arteries, but in many places pass under 

 them, when the course over them is as direct, it would seem 

 probable, as some physiologists have suggested, that this was 

 done in order that they may have the pulsation of such arteries 

 communicated to them, which pulsation, inconsiderable as it is, 

 may rather promote the passage of the lymph. 



