180 A Manual of Veterinary Physiology. 



process such as is assumed by some observers, who have 

 not been able to trace any communicating channels, paths, 

 nicks, or crannies between the lymph-space and the capil- 

 lary. 



The lymph having now reached the capillaries from the 

 spaces, we have next to consider how it is pressed onwards 

 so as to reach the thoracic duct. 



The Movements of the Lymph are due partly to the blood 

 pressure in the arteries at the seat of the formation of 

 lymph, which, with certain factors now to be mentioned, 

 forces the fluid from the lymph-spaces to the thoracic 

 duct. The pressure in the lymphatic of a horse has been 

 ascertained by Weiss to be -4 to 78 inch of mercury. 

 There is a gradual fall of pressure from the tissues to the 

 duct. Muscular contractions also mechanically favour the 

 passage of the lymph, the vessels for which are provided 

 with valves, which prevent the fluid from flowing backwards: 

 the obstruction caused by the lymphatics passing through 

 the various glands must be considerable, yet these latter, 

 by means of the contraction of their covering of involuntary 

 muscular fibre, more than compensate for the obstruction 

 caused by the gland itself. Once the lymph has found its 

 way into the thoracic duct, its passage into the general 

 circulation is not only favoured by gravity, but also by the 

 negative pressure produced in the jugular vein by the 

 process of inspiration, the result being that the lymph is 

 aspirated out of the duct into the vessel. This aspirating 

 influence has been proved by experimental inquiry, a 

 negative pressure having been observed during inspiration, 

 though a positive pressure of *5 inch of mercury exists in 

 the thoracic duct of the horse during expiration. That 

 this is not due to the positive pressure in the jugular vein 

 is certain, for the latter is prevented from forcing blood into 

 the thoracic duct by the presence of a valve, which only 

 allows fluid to pass from the duct into the vein. 



The lymph moves slowly in its vessels. WYiss has ob- 

 served !» inches to 11 inches per minute in a largo lyui 

 phatic in the neck of a horse. 



