Chap, xiii.] CIRCULATION. 207 



it 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.^ 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 

 hundredths 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, &,fter an excitation of a 

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

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

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

 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 

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

 speak of them at tolerable length. Their action determines the 

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

 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 

 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, Woehmschrijifiir die gesammUe Heilkunde, 1840 (No. 2^). 



