CHAP. XXVIII.] THE CIRCULATION IN THE ARTERIES. 351 



we must not seek for the operation of other forces in order to ex- 

 plain its phenomena. To determine these points we must investigate 

 the phenomena of the circulation in each of the systems of blood- 

 vessels, and first in the arteries. 



Phenomena of the Circulation in the Arteries. By each contrac- 

 tion of the left ventricle a certain quantity of blood is pumped into 

 the arterial system which is already full. Were the arteries and 

 other blood-vessels a series of rigid and inelastic tubes, there would 

 necessarily ensue upon this a discharge of blood, corresponding in 

 quantity and rapidity, from the opposite extremity of the system. 

 It is plain, however, from the slow rate of the venous circulation, 

 and the less capacity of the auricles as compared with the ventricles, 

 that this does not take place in the vascular system ; nor, consider- 

 ing the great extent of surface which the blood has to travel over 

 in the capillaries, and the consequent friction it has to encounter, 

 can it be expected that a quantity of blood should be discharged 

 into the capillaries equal to that which the heart injects into the 

 arteries. 



Room is obtained for each fresh quantity of blood (beyond that 

 which can be simultaneously expelled from the opposite extremity 

 of the vascular system) by the dilatation of the arteries under the 

 force of the heart. The eminently extensible and elastic character 

 of the arterial walls, thus gives a peculiar feature to the arterial 

 circulation, and is turned to good account in maintaining the flow 

 in that system of blood-vessels. In yielding under the force of the 

 heart, the arteries become dilated at each systole to the extent, 

 according to the experiment of Poiseuille upon the carotid of a 

 horse, of one-twenty-third of its diameter, or of one-twenty-second, 

 in a similar trial by Valentin on the carotid of a dog ; but which 

 must vary in different arteries and at different times with the force 

 of the heart, and the extensibility of the arterial wall. Poiseuille's 

 observation, however, pointed out unequivocally the fact (pre- 

 viously doubted), that the arteries are dilated at each systole of 

 the heart.* 



This dilatation of the arteries calls into play a force which in 

 some degree replaces the heart's force. The elastic arterial wall, 

 stretched by the contraction of the heart, reacts with a power which 

 approximates more closely to that by which it was dilated accord- 

 ing as the arterial tissue is more or less elastic. The arteries are 

 thus made to contract upon their contained blood, and to drive it 



* See the account of Poiseuille's experiment, and a figure of his instrument, 

 in Majendie's Journal, tom.ix., and also in Valentin' s\Physiologie, Bd.i. p. 449. 



