468 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



the large venous trunks into the veins of the thorax, and by the dilata- 

 tion of the right auricle from thence into the heart. As the lymphatics 

 empty into the veins in the neighborhood of the heart, this aspiration 

 will also be exerted on the lymphatics and will tend to produce a nega- 

 tive pressure. The lymph will, consequently, be subjected to a steadily 

 decreasing pressure from the periphery to the central vessels, and will, 

 therefore, move from the lymphatic radicals toward the venous trunks. 

 The onward motion of the lymph is also facilitated by muscular move- 

 ments, which, by compressing the lymphatics, force their contents onward, 

 backward motion being prevented by numerous valves. 



The lymphatic vessels, also, possess the power of rhythmic contrac- 

 tion, through the contraction of their muscular fibres, and sometimes 

 true lymph-hearts aid the propulsion of the lymph. 



In certain organs special mechanisms are concerned in the propul- 

 sion of the lymph. Thus, in the abdominal surface of the central tendon 

 of the diaphragm there are free communications between the peritoneal 

 cavity and the lymphatics of the diaphragm; and as the central tendon 

 is composed of two layers of fibrous tissue arranged in different 

 directions, these layers are alternately pressed together and pulled apart 

 in the respiratory movements of the diaphragm. The effect is to pump 

 lymph into the spaces between these layers. A similar mechanism exists 

 in the costal pleura and in the fascia covering the muscles. " When a 

 muscle contracts, lymph is forced out from between the layers of the 

 fascia, while, when it relaxes, the lymph from the muscle, carrying with 

 it some of the waste products of muscular action, passes out of the 

 muscle into the fascia, between the now partially separated hvyers " 

 (Landois). While the lymph-glands offer considerable resistance to the 

 onward passage of the lymph, this is, to a certain extent, compensated 

 by the non-striped muscular fibres which exist in the capsule and 

 trabecule of the glands, these muscles, together with those of the 

 lymphatics and lymphatic hearts, when present, being directly under the 

 control of the nervous system. 



The velocity of the lymph-current increases as the trunk increases 

 in size, from the decrease in sectional area. In the large lymphatic in 

 the neck of the horse it has been placed at two hundred and thirty to 

 three hundred millimeters per minute ; it must, therefore, be very slow 

 in the small vessels. The lateral pressure in the lymphatics in the neck 

 of the horse has been estimated at from ten to twenty millimeters of a 

 weak soda solution (Weiss). 



