420 THE NERVOUS REGULATION OF THE BLOOD-VESSELS 



low strength and frequency are employed. In fact, pressor and de- 

 pressor fibers are contained in many nerves, such as the sciatic and the 

 vagus, but their presence can only be detected by the stimulation of 

 the central ends of these nerves and by the use of infrequent shocks of 

 low intensity. 



Methods Used to Detect Vasomotor Action. While it has been 

 possible to prove histologically that the walls of the blood-vessels 

 contain nervous structures, this fact in itself is not sufficient to show 

 that they are in possession of vasomotor elements. In other words, 

 the only definite proof of vasomotor activity is to be found in the oc- 

 currence of the reaction itself. We may resort to inspection, because 

 if all indirect factors, such as external pressure, have been ruled out, 

 the blanching of a part may justly be referred to a vasoconstriction 

 and its reddening to a vasodilatation. These alterations in the 

 vascularity are usually associated with changes in temperature, a vaso- 

 constriction occasioning a fall and a vasodilatation a rise in the tem- 

 perature of the part. Probably the most direct proof of vasomotor 

 activity may be obtained with the help of the recording stromuhr, 

 this instrument being inserted in the artery or vein of the part to be 

 experimented upon. As has been stated above, this instrument 

 registers the volume of the blood stream and may therefore be used 

 to see whether or no the flow is affected by the excitation of nerves and 

 other experimental procedures. A decrease in the arterial supply 

 would then betray a constrictor action, and an increase a dilator 

 effect. It is also permissible to detect these vasomotor changes by 

 making a simultaneous record of the pressure in the artery and vein 

 of the organ to be examined. A mercurial manometer and water 

 manometer are employed for this purpose. Clearly, a rise in the 

 arterial and a fall in the venous pressure would betray a vasoconstric- 

 tion, and a fall in the arterial and a rise in the venous pressure, a 

 vasodilatation. These changes are easily explained, because the 

 former reaction must increase and the latter decrease the resistance 

 to the arterial throughflow. The manometer is also used to detect 

 vasomotor effects of a more general kind. It is then connected with 

 one of the principal arteries, such as the carotid or femoral. A rise in 

 the general pressure may then be attributed to a constriction of an 

 extensive area of the circulatory system, and a fall in the general 

 pressure to a vasodilatation of rather wide extent. Lastly, it js pos- 

 sible to place the organ to be experimented upon in a plethysmograph. 

 Under this condition a diminution in the volume of the organ would 

 point toward a vasoconstriction, and an increase in its volume toward 

 a vasodilatation. But naturally, if these procedures are practised, 

 care must be taken to exclude all indirect effects, such as may be pro- 

 duced by a mechanical obstruction to the blood flow. An occurrence of 

 this kind usually leads to a stagnation of the blood and an increase in 

 the volume of the organ which can scarcely be differentiated from a 

 true vasomotor effect. 



