CONTRACTILE TISSUE OF ARTERIES. 3 



somewhat modified, principally on account of the investigations of 

 Henle,* which have shewn that besides elastic tissue, the middle arterial 

 coat is provided with fibres in all respects analogous to those of organic 

 muscle. 



This discovery is one of considerable importance, because it serves to 

 explain what was hitherto unintelligible, the well-known property possessed 

 by arteries, especially by the smaller ones, of contracting, under certain 

 circumstances, to a diameter smaller than that which their elasticity ulone 

 could enable them to assume. Although this property has been matter of 

 almost universal observation, yet by few writers has any plausible explana- 

 tion of it been suggested. The sagacity of John Hunter, unaided by 

 microscopic evidence, led him to conclude that the contraction of arteries 

 was really a muscular act.f Yet this opinion appears to have been lost 

 sight of, for most writers since Hunter's time, including Professor Miiller, 

 have attributed the contraction of the arterial coats to a peculiar vital pro- 

 perty, to which they gave the name of tonicity or insensible contractility, 

 without being able to refer this property to any definite structure. In the 

 last German edition of the first volume of his work,J Professor Miiller 

 alludes to this discovery by Henle, and considers it probable that the fibres 

 described by him are the seat of the contractile power of the arteries, 

 though he appears disinclined to admit their muscular nature. 



Chemical evidence also in favour of Henle's account has been procured 

 by Dr. Retzius, who finds that a solution of the arterial coat in acetic acid 

 is precipitated by cyanide of potassium ; this shews that some elements 

 besides cellular and elastic tissues enter into its composition, for the 

 solution in acetic acid of neither of these tissues is precipitated by cyanide 

 of potassium. And that organic muscular fibre constitutes one of these 

 other elements has, since then, been rendered tolerably certain by Dr. F. 

 C. Donders,|| who, by acting upon the middle arterial coat of young 

 sheep with a solution of potash, observed that in two or three hours the 

 fibres of this coat separated from each other, became granular and then dis- 

 solved ; changes exactly similar to those which he found organic muscular 

 fibres, under like circumstances, to undergo. The existence of muscular 

 fibres in the middle arterial coat is also inferred by the same physiologist ^[ 

 from the effects produced upon it by the action of nitric acid and ammonia. 

 When strong nitric acid is applied to a protein-compound, it forms with 



* Casper's Wochenschr. May 28, 1840 ; and Allgem. Anat. 1842, p. 498. 



t Works, Palmer's edition, vol. iii. pp. 15671, comprising his account of the structure 

 of arteries, where (as Mr. Paget has pointed out) abundant proofs are given by Hunter of 

 the existence of muscular power, especially in the smaller arterial branches. 



I 1841, page 171. 



$ Physiol. des Mensch. by Prof. J. Miiller, Coblenz. Fourth edition. Vol. i. p. 17K 



|| Holliindische Beitrage zu den Anat. und Physiol. Wissenschaften, 1846, p. 55. 



1 Op. cit. p. 67. 



B 2 



