VASOMOTOR SYSTEM 115 



the flow in the veins and to hinder it in the arteries. The 

 flow in the latter, however, is not affected much on account of 

 the thickness of the arterial walls, so that, on the whole, the 

 negative pressure in the thorax, increased with each inspira- 

 tion, helps the pulmonary circulation. The capillaries are 

 situated so close to the surface that they are exposed to atmos- 

 pheric pressure. Every expiration presses the blood out of 

 them, and so again the flow is favored. 



Vasomotor System. The heart pumps the blood through 

 all parts of the body, but the amount in any one portion depends 

 upon the active dilatation or contraction of the vessels. That 

 an artery may dilate was first shown by Bernard, who cut the 

 cervical sympathetic of a rabbit on one side and found an 

 increased redness of the skin of the ear and an elevation of 

 the temperature of from four to six degrees, which persisted 

 for months. If the peripheral end of the cut nerve is stimulated 

 with a galvanic current, normal conditions are resumed, which 

 last only as long as the stimulus is applied. The existence of 

 dilator nerves is placed beyond doubt by the results obtained 

 upon the chorda tympani of the submaxillary gland. Nerves 

 that bring about a dilatation of bloodvessels are called vaso- 

 dilator nerves; those that cause a constriction are called vaso- 

 constrictor nerves. Both are present in the nerves of the sympa- 

 thetic system, as well as in the cranial and spinal nerves. 

 They also supply veins. The portal vein, for instance, may 

 be made to contract by stimulation of the peripheral end of 

 the cut splanchnic. The changes in the capacity of the blood- 

 vessels may be studied by direct inspection in many cases, 

 but often it is more satisfactory to place a manometer in a 

 branch of the artery that supplies the portion of the animal 

 under observation. The principle underlying this is that the 

 pressure in an artery depends upon the resistance to be over- 

 come in its distal capillaries. Another method of studying 

 vasomotor phenomena is to enclose a portion within an air- 

 tight cylinder, which usually is filled with a liquid and is con- 

 nected with a tambour. Changes in volume of the parts 

 enclosed, due to variations in the amounts of blood, are trans- 

 mitted to a tambour. Such an instrument is called a plethysmo- 

 graph. 



Vasoconstrictor and \ ;i>odilator nerves are usually found 



