The coats of the smaller arteries are generally believed to be proportionally thicker than those of the larger trunks, and John Hunter held the opinion that the yellow fibrous tissue exists in greatest quantity in the larger arteries ; while the smaller vessels, considered more active, are composed of a substance more nearly allied to muscular fibre. The grounds upon which the latter opinion rests are upon the whole not very satisfactory; and it appears to be opposed by those instances in which, after the closure by ligature of the principal artery of a limb, the smaller collateral vessels which maintain the circulation, after undergoing a rapid enlarge ment, assume the structure and general appear ance of the large arteries.
The irritability of the smaller arteries, now very generally admitted by physiologists, though it seems by some to have been inferred fi.oin analogy, and to have been rendered probable by Dr. Wilson Philip's observations on the effect of chemical stimuli in removing the dilated state of the capillaries in inflammation, was first distinctly proved experimentally by Dr. Thomson of Edinburgh,* who caused the arte ries in the web of the frog's foot to contract powerfully by the application of mechanical irritation as well as by chemical stimuli. His experiments shewed that the nature of the con traction produced by stimulation of one of the smaller arteries varies considerably, occupying sometimes a greater or less space of the vessel, and being at other times confined to one place, sudden, and frequently so great as completely to stop the passage of blood. They also de monstrated the fact that the contraction of the small arteries does not follow immediately the application of' the stimulus, as occurs in the voluntary muscles, but that a period of from one to three minutes elapses before the contrac tion begins, and that the vessel remains con stricted for some time, and then returns to its original state, unless inflammation shall have occurred, in which case it dilates to a greater size than natural. The irritability of the small vessels has been fully established by experi ments similar to those of Dr. Thomson, by Dr. Wilson Philip,t Dr. Hastings,t Kaltenbrun ner,§ and Wedemeyer,§ the last of whom suc ceeded in causing the small arteries to contract by means of galvanic as well as of mechanical irritation. The constriction which .follows the injection of styptic and irritating fluids into the arteries, observed by Halesil in animals recently dead, and similar experiments by Wedemeyer, may be adduced as another proof of their irrita bility. The stoppage of hemorrhage from cuts
of the small arteries and capillaries, assisted as it is by cold or irritating applications, may be regarded as the effect of the same property.
Contractions do not occur so readily or ob viously in the large as in the very small arte ries. Verschuir appears to have been the first who observed, in a manner not liable to fallacy, distinct contractions of the larger arteries to occur after the direct application of a stimulus. From an extended series of experiments upon this subject, described in his Inaugural Disser tation De Vi Arteriarum Contractili, Verschuir was led to adopt the opinion that the arteries are possessed of irritability, or contract in the same manner as muscles do from irritation ; as he observed very obvious and powerful con tractions to occur when, by means of a sharp point or chemical stimuli, he irritated the coats of the larger arteries of animals.
Ilaller, though considering the middle coat of the arteries as of a muscular nature, was un successful in producing obvious contractions in them. The repetition of the experiments of Verschuir by many others has been attended with very various results; some confirming his observations, others having entirely failed in producing any obvious contraction, or not being disposed to consider it of a muscular kind. Among the last may be mentioned Nysten, Bichat, Wedemeyer, and J. Miiller.
It must be obvious that, laying aside the difference of opinion regarding the nature of the contractions when they are admitted to occur, in a question of this kind a positive re sult deserves more consideration than a nega tive one, provided the phenomena stated to have been observed are such as to be appre ciable by all. Among the experiments favour able to the view that the large arteries are en dowed with irritability, may be mentioned those described by Hastings,* and a series of unpub lished observations by Dr. Thomson, to which we have access, which seem to prove in a very satisfactory manner the frequent occurrence of .contractions in the larger arteries after stimula tion; and to point out as a cause of the failure ..of some at least of the previous experiments, -the long time which commonly elapses between the application of the stimulus and the occur -rence of the contraction ; together with the cir -cumstances formerly remarked by Verschuir, that the contraction is not an invariable conse quence 'of the stimulation, and that it occurs much more readily in some animals than in others.