THE CIRCULATION. 



323 



tricity, phenomena that are opposite to those just detailed ensue. The 

 arterioles and capillaries become so contracted that they are no longer 

 visible ; the size of the organ supplied by these nerves diminishes ; the 

 venous blood becomes dark. If you cut the organ, less blood flows 

 out of it than when there is paralysis of the constrictors and, there- 

 fore, dilatation. 



The vasomotor nerves are always in a condition of antagonism, 

 although the constrictor influence is by far the more powerful. 

 Thus, if a nerve-trunk which contains both constrictor and dilator 

 fibers be stimulated, the first effect is constriction of the arterioles 

 and capillaries supplied by the artery. This condition of constriction 



Fig. 107. Effect of Irritation of the Splanchnic Nerve on the Aortic 

 Pressure. ( GLEY. ) 



Dog with medulla divided. Pr, lat. Ao, Aortic pressure. 8, Time in seconds. 

 E, Faradic irritation of the peripheral end of the left splanchnic. 



lasts for some time, but is eventually replaced by dilatation of the 

 vessels of the part. This dilatation is a sequel, and is to be ex- 

 plained by the fact that the vasodilator fibers are less easily exhausted 

 than the vasoconstrictor fibers. For, after separation of the vaso- 

 motor fibers from the central nervous system, it is found that the 

 vasodilator fibers do not lose their excitability before the lapse of 

 from six to ten days. The vasoconstrictor fibers do not respond to 

 excitation after the third or fourth day. 



Vasoconstrictors of the Head. It is known that the cervical 



sympathetic is the vasoconstrictor for the corresponding side of the 



face, ear, cheeks, lips, brow, eye, middle-ear, and tongue, with the sub- 



mxillary and parotid glands; in fact, all parts of the head with 



ie exception of the brain are supplied by it. The vasoconstrictors of 



