296 CIRCULATION OF THE BLOOD. 



aspect. 1 Soon the discharge from the thoracic duct is reddened 

 by the alkanet. It has been shown by Ludwig that in the ex- 

 tremities, the tendons and aponeuroses are the special seat of 

 the net-works of capillaries by which the lymphatics commence, 

 and that the}' have here an arrangement similar to that observed 

 in the central tendon of the diaphragm. The experiment proves 

 that even passive movements of the limbs, by alternately tight- 

 ening and relaxing these structures, press forwards the lymph 

 stream. The influence of active movements must be much 

 greater. 



85. Internal Absorption by the Veins. The propo- 

 sition stated at the beginning of the section, that substances 

 in solution enter the capillaries from the tissues by a process 

 of absorption, which is under the immediate control of the 

 nervous system, may be strikingly illustrated as follows : 



Two frogs having been slightly curarized are prepared thus: 

 The heart having been exposed lege artis ; a small opening is 

 made in the skin in the occipital region. In one of the frogs, 

 the brain and spinal cord are completely destroyed by passing 

 a needle upwards and downwards from the occipital region, 

 and then both are hung vertically on a board, side by side, 

 looking in the same direction. A small funnel, the stem of 

 which is drawn out into a narrow beak, is now passed from the 

 incision downwards under the skin of each animal, till its end 

 reaches the dorsal lymphatic sac. This done, the bulbus 

 aortae is divided in both animals, and the results are observed. 

 In the frog deprived of its central nervous system, only a few 

 drops of blood escape the quantity, that is to say, previously 

 contained in the heart and in the beginning of the arterial 

 system. In the other, the bleeding is not only more abundant, 

 but continues for several minutes after the section. As soon 

 as bleeding lias ceased, a quantity of saline solution (say, 5 to 

 10 centimetres) is injected into the lymphatic sac of each animal 

 until it is distended, and the exact quantity used carefully 

 noted. In the frog in which the central nervous system is 

 intact, the discharge of blood from the opening in the bulb 

 begins again, and goes on increasing; while the liquid, which 

 at first is nearly pure blood, becomes more and more diluted 

 with serum. The discharge of sanguineous liquid goes on for 

 one or two hours ; and if, during the progress of the experi- 

 ment, the vasomotor centre is stimulated reflexly by exciting 

 a sensory nerve or the surface of the skin, it is seen that the 

 rate of flow is at once augmented, but becomes less after the 

 cessation of the excitation than it was before. This last fact 



1 Colored drawings of the injections so obtained will be found in Lud- 

 wig and Schweigger-Seidel's beautiful monograph on the lymphatics of 

 tendons and aponeuroses. 



