INFLUENCE OF THE NERVOUS SYSTEM. 171 



tremity of the cervical sympathetic nerve, when cut off, a 

 constriction of the auricular vessels occurs, and consequently, 

 a return to the normal temperature, or even, from anaBinia, a 

 lower temperature may follow. 



Since that time, the function (role) of the great sympa- 

 thetic, as a vaso-motor nerve, has been clearly demonstrated 

 in other parts of the body, the limbs, and the abdominal 

 viscera, as well as of the head. Kussmaul and Tenner con- 

 firmed the opinion that the calorific influence is entirely 

 vaso-motor, and Van der Beke Callenfels (1856) proved that 

 this afflux of blood in any part of the periphery which is 

 much exposed to radiation causes considerable loss of heat 

 in the animal. 



Experimental physiology of the great sympathetic as a 

 vaso-motor nerve, may now be pursued by studying the 

 effects produced by its section and excitation, as has been 

 done by Mons. Legros in his monograph : 1. Section of a 

 sympathetic branch is instantly followed by the paralyzation 

 of the smooth muscles innervated by this branch, especially 

 the muscles of the vessels: the small vessels are seen to 

 dilate, and the capillary network to fill, on account of the 

 increased afflux of blood. Generally it may be easily ob- 

 served in a rabbit's ear, for instance, that vessels which were 

 hardly visible before the operation, can be distinctly seen 

 after it. In short, passive hyperamia takes place. 2. By 

 bringing an induced current of electricity to bear upon the 

 peripheric extremity of the sympathetic nerve, after section, 

 quite a contrary phenomenon is produced: the vascular 

 muscles contract, the vessels shrink, and active anemia 

 follows. If the excitation ceases, a marked dilatation suc- 

 ceeds. The capillaries are entirely passive during all these 

 phenomena: the whole process takes place in the small veins 

 and arteries. The essentially passive part played by the 

 capillaries, during the alternations of contraction and relaxa- 

 tion going on in the vessels, is best understood by studying 

 the pressure or vascular tension which accompanies experi- 

 ments on the vaso-motor nerves. If, indeed, the capillaries 

 dilated, as do the arteries, after section of the sympathetic 

 nerve, the flow of blood would be increased, but the resist- 

 ance would be less, and the pressure lower. On the other hand, 

 if excitation of the sympathetic nerve caused the capillaries 

 to contract as do the other vessels, the resistance would in- 



1 Ch. Legros, " Des nerfs vaso-moteurs." Paris, 1873. 



