82 VALVES IN THE VEINS. 



running much faster than the blood current. To prove this 

 point, make the following experiment : Pour a little red ink 

 into a dish of water, and while working the bulb of the syr- 

 inge as in Experiment 4, quickly transfer the supply tube 

 to the colored liquid. The pulse will continue unchanged, but 

 it may take several strokes before the colored liquid issues 

 from the nozzle. 



The pulse may be compared to the shock that runs through 

 a freight train. When the engine bumps against the first car, 

 this car strikes the next, and so on through the whole length 

 of the train. The shock has gone the length of the train, but 

 no car has moved more than a few feet. 



The Veins. The capillaries, after penetrating the tissues, 

 reunite to form small veins, till finally two great veins, the 

 Caval Veins, Precaval and Postcaval, return the blood to the 

 heart. The veins, like the arteries, are smooth inside and 

 elastic (though less elastic than the arteries). They are 

 thinner than the arteries, and, in consequence, collapse when 

 the blood flows out of them, whereas the larger arteries stand 

 open, after they are emptied of blood. 



The Valves in the Veins. The only valves in the ar- 

 teries are those which we have seen at the beginning of the 

 aorta and pulmonary artery. Many of the veins have similar 

 pocket-like valves, though lose strong than those of the ar- 

 teries. Dissect back the skin from the throat of the cat or 

 the rabbit, till the Jugular veins are well exposed. Let the 

 head of the animal hang over the edge of the table, and ob- 

 serve that as the blood passes toward the head it causes a 

 marked bulging at certain points. With the handle of the 

 scalpel gently stroke the vein toward the head, watching 

 these swellings. Dissect out the vein from the head to the 

 shoulder. Insert the nozzle of a syringe, first into one end, 

 then into the other, and note the effect of sending a stream 



