210 BIOLOGY. [BOOK n. 



We can now form a sufficiently exact idea of the arterial 

 circulation. At every ventricular systole a certain quantity of 

 blood, on an average fifty grammes in man, is driven from the 

 heart into the arteries. The valves situated at the orifice of the 

 arterial trunks in the heart, the sigmoidal valves, rise to let the 

 flow pass, and afterwards fall to hinder the reflux. The elasticity 

 and the contractility of the arteries, vanquished by this sudden 

 afflux, yield ; the calibre of the vessels abruptly augments ; but 

 the muscular fibro-cells of the arterial walls, excited by their 

 vaso-motory nerves, instantly re-act ; the artery contracts, and 

 impresses on the sanguineous wave a new impulsion. Naturally 

 it is the large trunks, those which receive the shock the most 

 directly, that dilate the most ; but the sanguineous impulsion is 

 expended in part for this dilatation ; the arterial contraction 

 which it has provoked comes to its aid, is a supplement thereto 

 in a certain measure, and it acts with the more continuity the 

 more distant the arterial vessel is from the heart, and is influenced 

 less by the sudden effort of the cardiacal impulsion. Thus in th 

 fine arterial ramifications the sanguineous current is perceptibly 

 continuous and uniform ; dilatation and pulsation scarcely exist 

 in them. This is why we observe in these small vessels a con- 

 siderable diversity of operation. Of two arterial ramuscules 

 emanating from a common trunk, one is the seat of a rapid cir- 

 culation, the other of a slow circulation. Other differences seem 

 to depend on the organs, in the tissue whereof the arteries pro- 

 ceed to distribute themselves. If these organs are the seat of a 

 great nutritive expenditure, of important material exchanges, the 

 arterial contractions are the more energetic; for they must 

 maintain a more active sanguineous current. Also the arteries 

 of the brain, of the marrow, of the glands obey the artificial 

 excitations better than tha others ; they contract more rapidly 

 and more vigorously. 1 



The circulation varies also in a general manner in the different 

 parts of the arterial tree ; the course of the blood slackens in the 

 1 Vulpian, loc. cit., p. 64. 



