OF THE ARTERIES. 285 



423. The function of the arteries is to convey the Wood 

 from the heart to all the parts of the bod}-. When the ventri- 

 cles of the heart propel, while contracting, a new supply of li- 

 quid into the arteries, already full with blood in motion, the 

 velocity of the motion is increased in all the arteries: the ob- 

 servation of the wound of an artery proves it. Another effect 

 of the systole of the ventricles, generally admitted, is the dila- 

 tation of the arteries. Experiments have been cited in sup- 

 port of this dilatation; other interesting experiments of Doctor 

 Parry appear to contradict it; it really exists, however, but it 

 is very inconsiderable. Another effect, but more appreciable, 

 produced by each systole, is the elongation of the arteries. 

 The action exercised by the arteries in order to send forward 

 the blood, is their elastic return on themselves, which narrows 

 and shortens them, and consequently diminishes their capacity, 

 and moreover, a force of vital contraction which is added to 

 elasticity in the middle sized arteries, and this vital contrac- 

 tion exists exclusively in the small. The velocity of the course 

 of the arterial blood generally diminishes from the trunks to 

 the last ramifications; this velocity presents besides local va- 

 rieties, either permanent or accidental. 



The function of the arteries is therefore to convey, like ca- 

 nals, the blood in all the parts of the body, and like contrac- 

 tile canals, to communicate to it a part of the motion with 

 which it is animated. The action of the arteries on the blood 

 has been at times exaggerated and at others too little appre- 

 ciated. It is very certain, 1st, that the vessels appear before 

 the heart, both in the animal series and embryo; 2d, that the 

 monstrous foetus without a head is deprived of a heart; 3d, that 

 in fishes there is no aortic ventricle, and that even in man the 

 vena portarum (sect, iii.) is equally deprived of a muscular 

 agent calculated to communicate an impulsion; 4th, that in the 

 reptiles from which the heart has been removed, the motion 

 of the blood continues for a long time. All these facts evi- 

 dently prove that the vessels are an agent, and are even the 

 primitive agent of the motion of the blood. The arteries par- 

 ticipate in this movement by their elasticity and irritability. 



But it is no less certain, that in animals provided with a 



