THE VASOMOTOR MECHANISM. 131 



1840. In this year Henle 1 established the existence of a plentiful supply of 

 involuntary muscle cells in the middle coat of the small arteries, and thus laid 

 the anatomical foundation of the doctrine that while the general circulation of 

 the blood depends upon the heart, its distribution to any one part depends on 

 the calibre of the arteries supplying that part. The final link in the chain of 

 anatomical evidence was not wanting, for many observers had traced filaments 

 of nerves along the course of the arteries as far as their ultimate ramifications 

 in the tissues. To these nerves Stilling 2 gave the name " vasomotor," 

 ascribing to them the function of maintaining the tone and ordering the 

 contractions of the capillaries, arteries, and veins. 



The exact discovery of the vasomotor mechanism, a discovery fraught 

 with the utmost importance to the advance of physiological and patho- 

 logical knowledge, dates from Claude Bernard's observations on the 

 cervical sympathetic nerve of the rabbit, published in 1851. 3 He found, 

 after dividing this nerve, or excising the superior cervical ganglion, a 

 noticeable increase in the vascularity and the temperature of the ear on 

 the same side as the lesion. 



In such case the temperature may be raised some 4° to 6° C above 

 that of the opposite ear. Bernard was at first inclined to believe that 

 the sympathetic nerve, apart from its inHuence on the blood supply, 

 exerted control over the production of heat. 4 



In 1852, Brown-Sequard crowned Bernard's discovery by the publica- 

 tion of this observation : " If galvanism is applied to the superior portion 

 of the sympathetic, after it has been cut in the neck, the vessels of the 

 face and of the ear after a certain time begin to contract ; their con- 

 traction increases slowly, but at last it is evident that they resume 

 their normal condition, if they are not even smaller. Then the tempera- 

 ture and the sensibility diminish in the face and the ear, and they 

 become in the palsied side the same as in the sound side. When the 

 galvanic current ceases to act, the vessels begin to dilate again, and all 

 the phenomena discovered by Dr. Bernard reappear." 5 



Unwitting of the fact that Brown-Sequard had forestalled him, 

 Bernard 6 himself discovered that excitation of the cervical sympathetic 

 nerve produced constriction of the blood vessels of the ear. The same 

 conclusion was arrived at independently by Waller. 7 



The amount of dilatation in the ear which follows section of the 

 cervical sympathetic nerve is a sign of the strength of vascular tone. 

 It must be borne in mind that dilatation is not always produced thereby, 

 seeing that the tone of any particular vascular area is constantly varying. 

 Thus, if the rabbit be placed in a warm atmosphere, division of the 

 cervical sympathetic will induce but little change in the vascularity 

 or temperature of the ear ; while, if the atmosphere be cold, the tempera- 

 ture of the ear may rise after the operation 5° C. or even 10° C. 



Schiff made a further discovery of great importance. After dividing 

 one cervical sympathetic nerve, he found, on exciting the animal, or taking 

 it for a run in the warmth of the sun, the ear on the uninjured side 

 became slightly the warmer, and the vascularity of the ear more pro- 



1 " Allgemeine Anatomie," Leipzig, 1841, S. 525, 690. 



2 "Physiol. Untersuch. ueber die Spinalirritation," 1840, S. 163, 275. 



3 Compt. rend. Soc. de biol., Paris, 1851, p. 163. 



4 Compt. rend. Acad. d. sc, Paris, 1852, tome xxxiv. pp. 472-475. 



5 Med. Exam., Pliila., Aug. 1852, p. 490. 



6 Compt. rend. Soc. de biol., Paris, 1852, p. 169. 



7 Compt. rend. Acad. d. sc, Paris, 1853, tome xxxvi. p. 378. 



