t 3 2 THE MECHANISM OF THE CIRCULATION. 



nounced. So soon, however, as the animal became quiet these condi- 

 tions were once more reversed. Schiff explained this result by suppos- 

 ing the existence of vaso-dilator nerves. 1 



The existence of vaso-dilator nerves, thus foreshadowed by Schiff, 

 was conclusively proved by Bernard. 2 Exposing the submaxillary gland 

 of a dog, he opened one of the efferent veins, and observed the outflow 

 of blood. On dividing the cervical sympathetic nerve, the blood flowed 

 in increased volume and became more arterial in colour. On exciting the 

 same nerve, the outflow entirely ceased, and the gland became pale in 

 colour. He next excited the chorda tympani nerve, the gland blushed 

 red, and almost immediately bright arterial blood gushed out from the 

 vein. The vascular dilatation was in this case so great that the blood, 

 with each pulsation of the artery, flowed from the vein in jets. 



By this fundamental experiment, Bernard proved that there exist 

 two kinds of vasomotor nerves. The one, the vaso-constrictors, which 

 exert a tonic action, for on section of these the gland blushes ; the other, 

 the vaso-dilators, which exert no tonic action, but can, when excited, 

 throw the vessels into the widest dilatation. 



Evidence that the vaso-constrictor nerves have their origin in the 

 spinal cord was afforded by Schiff', who found that vascular dilatation 

 and a rise of temperature in various parts of the body followed destruc- 

 tion of certain parts of the cord. 



Budge, 3 by unilateral extirpation of the cord between the last 

 cervical and third thoracic nerve, obtained the same vaso-dilatation as is 

 seen after section of the cervical sympathetic. Waller 4 constricted the 

 vessels of the ear by excitation of the cervical cord. The vasomotor 

 fibres issue by the anterior roots. The first observation of this fact (on 

 the frog) is attributed to Pttuger. 5 



These experiments of Bernard, Brown-Sequard, Waller, and Schiff 

 have led to a vast amount of inquiry into the physiological action and 

 anatomical distribution of vasomotor nerves. 



The methods which have been employed are these : — 



1. Direct observation of the colour of any part, or of the amount of blood 

 which flows from a wound. 



This method has been used with advantage in the case of the rabbit's ear, 

 the submaxillary gland, the mucous membrane of the mouth. Microscopical 

 observation and measurement of the calibre of the arterioles has been employed 

 in investigations of the frog's web, mesentery, mylohyoid muscle. 



2. Thermometric observations of the temperature of any organ. This is 

 best carried out by means of thermo-electric junctions. 



3. Estimation of the output of blood per minute from the efferent vein of 

 an organ. 



4. Records of the velocity of blood flow in the artery supplying an organ. 

 This can be effected either by using Chauveau's dromograph or Cybulski's 

 photohsematochometer. 



5. Estimation of the changes in volume of an organ, such as the spleen, 

 the kidney, the brain, the lung, the intestines, or the limbs. This is effected 

 by means of the plethysmograph. The organ is placed in an air-tight box, 

 and this in its turn is connected with a recording tambour. 



■.-> 



1 Berner Schriften, 1856, p. 69; " Untersuch. ueber Zuckerbildung in der Leber," 

 Wiirzburg, 1859, S. 153-156. 



- Journ. de laphysiol. de Vhomme, Paris, 1858, tome i. pp. 237, 649. 



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



4 Ibid., p. 381. 



5 Allg. med, Centr.-Ztg., Berlin, 1855. 



