CHAP. XXVIII.] VELOCITY OF THE ARTERIAL CIRCULATION. 363 



in the column of blood to a variable extent at and after each pulse. 

 This observation has been confirmed by Ludwig, and also by Volk- 

 mann. The rise corresponds to the heart's systole, the fall to the 

 diastole. This affords, in the greatest part of the arterial system, 

 the clearest proof of the extension of the heart's influence through- 

 out it. And Poiseuille showed that the pulsations of the heart could 

 be counted by noticing the advance of the blood in pulses along the 

 capillaries, and, under certain circumstances, even along the small 

 veins. 



Influence of Respiration That respiration exercises an influence 

 upon the circulation has been likewise noticed by several observers. 

 Hales had referred to this ; he had noticed how the straining 

 efforts of the animals which were made the subjects of his ex- 

 periments, were followed by a rise of the column of blood in the 

 tube, and how the same effect followed deep sighing. And Poi- 

 seuille found that during expiration the height of the column of 

 mercury was much increased, but that it fell in inspiration. In 

 forced and deep inspirations the force of the heart becomes so much 

 diminished in some cases that no pulse, or at most a very feeble one, 

 can be felt at the wrist : on the other hand, in forcible expirations, 

 the pressure of the blood in the arteries becomes double its normal 

 amount. This has been further confirmed by Ludwig and Volk- 

 mann. The fact is of practical interest, and affords good reasons 

 why the practitioner should caution those whose arteries are weak- 

 ened by a diseased state of their tunics, against strong efforts, or 

 against any action likely to disturb the quiet and freedom of the 

 breathing. 



The heart's force is materially weakened, as Blake's experiments 

 show, by the introduction into the circulation of poisonous agents 

 of a sedative nature. And there is every reason to believe, that 

 the existence of particular animal poisons in the blood, as the 

 typhus poison, that of scarlet fever, of erysipelas, etc., is capable of 

 depressing the heart and weakening the circulation. 



Thus then, as far as regards the powers by which the blood is 

 moved in the arteries, it may be stated that the circulation is main- 

 tained in them by the force of the heart ; replaced and propagated 

 throughout the system by the elastic reaction of the arterial tunics, 

 and to a certain extent restrained or modified by the muscular con- 

 traction of the same tunic, which likewise serves very accurately to 

 adapt the size of the arteries to the quantity of blood contained in 

 them. 



On the Velocity with which the Blood moves in the Arteries. 



BB 2 



