THE CIRCULATORY SYSTEM 169 



a rabbit, for instance, the blood will rise in the tube to a 

 height of about three feet, and will be raised slightly 

 farther at each beat of the heart. If a similar tube be 

 placed in an opening in the jugular vein, the blood will 

 rise in the tube only very slightly, and the height will 

 not be affected by the heart beats. 



234. Velocity of the Blood. Of course the same quan- 

 tity of blood flows through the aorta as flows through all 

 the capillaries of the system and is returned to the right 

 auricle ; but all the capillaries together hold much more 

 blood than the aorta. The blood must, therefore, pass 

 more rapidly through the aorta than through the capil- 

 laries. Its rate is about fifteen inches a second in the 

 aorta, and about half that in the two venae cavie, while 

 in the capillaries it is thought to be less than one twenti- 

 eth of an inch in a second. 



235. The Lymphatic Circulation. We have seen ( 209) 

 that the plasma of the blood which oozes through the thin 

 walls of the blood capillaries forms the fluid called lymph. 

 This fluid contains corpuscles apparently identical with the 

 white blood corpuscles, but no red ones. As it fills the 

 spaces between the cells of the tissues it conveys nutri- 

 ment directly to the cells themselves in all parts of the 

 body. A network of delicate vessels called lymph capil- 

 laries carries away the surplus fluid. The walls of these 

 tubes are extremely thin, being composed, like the blood 

 capillaries, of a single layer of flat cells with a little con- 

 nective tissue and a few plain muscular fibers. The small 

 vessels unite to make up larger ones, which are supplied 

 with valves, like the veins, and all finally pour their con- 

 tents into two large tubes. One of these is called the 

 thoracic duct, and runs upward in the thorax and abdomen 

 along the spinal column to empty into the angle of June- 



