CHAP, xin.] CIRCULATION. 207 



tit is impelled into the general arterial system, whither we have 

 now to follow it. 



B. Arteries. The arteries, that is to say, all the vessels going 

 'by ramification from the heart to the capillaries, have all an 

 elastic and contractile wall. Long there was a controversy re- 

 garding the nature of the contractile elements of the arteries till 

 the moment, when in 1840, Henle demonstrated that the arterial 

 'contractility is due to anatomical elements identical with those 

 of the muscles of organic life. 1 They are fusiform fibro- cells 

 transversely placed relatively to the axis of the vessel, and fur- 

 nished with a nucleus. They have a length of from five to seven 

 Ihundredths of a millimetre, and have a breadth of from five to 

 : six thousandths of a millimetre on the level of the nucleus The 

 mode of contraction is naturally in strict relation with the 

 nature of these anatomical elements. The contractions of the 

 ; fibro cells is effected slowly, progressively, after an excitation of a 

 I certain duration. But in requital, they persist during from ten 

 i to fifteen seconds at the least, and the artery returns slowly to 

 | its primitive state. This muscular layer is in general the thicker, 

 the larger the calibre of the vessel is. It diminishes in the 

 degree of the ramification of the arteries, and ceases altogether in 

 ! the capillaries. 



The arterial contractility is less independent of the nervous 



system than that of the heart. The experimental researches of 



modern physiology have, in effect, demonstrated the existence of 



I motory nerves of the vessels, or vaso-motories. We shall have to 



I speak of them at tolerable length. Their action determines the 



paralysis of the vessels, which dilate ; for they then lose their 



j tonicity, the state of demi-contraction which keeps always the 



: arterial calibre in a certain degree of "narrowness. If we cut in 



, a rabbit's neck the cord of the great sympathetic nerve whence 



I emanate the vaso-motor nerves of the ear, we see ceasing the 



rhythmical beatings of the auricular artery, which normally are 



effected five or six times a minute. The excitatory nervous 



1 Henle, Wochenschri/l fiir die gesammte Hdlkunde, 1840 (No. 21). 



