EFFECTS OF MUSCULARITY OF ARTERIES. 329 



in great part upon the due tonicity of the muscular coat of the arteries 

 ( 365). When this tonicity is in excess, the walls of the arteries are 

 too rigid ; the pulse at the wrist is felt to occur exactly at the same 

 time with the ventricular systole ; and its character is that of strength, 

 incompressibility, and sustained power, though it may be even slower 

 than usual. This is the case in what is commonly termed "high condi- 

 tion" of the system; which predisposes to inflammatory disorders, but 

 which renders it less susceptible than usual to the influence of mala- 

 ria, contagious miasmata, or other causes of a depressing character. 

 On the other hand, when the tonicity of the arteries is less than it 

 should be, their walls yield too much to the pulse-wave ; so that the 

 pulse at the wrist is often felt even after the second sound is heard ; and 

 the pulse itself is jerking, unsteady, and too easily compressible. This 

 loose relaxed state of the vessels is the most unfavourable that can be 

 to regularity and vigour of the circulation; and it manifests its ill 

 effects in the general condition of the system, which is then peculiarly 

 prone to suffer from the agency of malaria, infectious miasmata, or 

 any other depressing causes. 



585. Although many Physiologists have denied that the Arteries 

 possess real Muscular Contractility in any degree, yet there can be no 

 longer any doubt on the subject ; since numerous experimenters have 

 succeeded in producing distinct contraction in their walls, by the appli- 

 cation of those stimuli which act upon muscular fibre in general. More- 

 over it has been ascertained, that when an artery is dilated by the blood 

 injected into it from the heart, it reacts with a force superior to the 

 impulse to which it yielded ; and that, if a portion of an artery from -an 

 animal recently dead, in which the vital properties are still preserved, 

 and a similar portion from an animal that has been dead some days, in 

 which nothing but the elasticity remains, be distended with equal force, 

 the former contracts to a much greater degree than the latter after the 

 distending force is withdrawn. One use of this contractile power may 

 very probably be, to assist the Heart in maintaining the flow of blood ; 

 for if the arterial walls yield readily to the ingress of blood, and then 

 contract upon their contents with a force greater than that which dis- 

 tended them, the current must necessarily be propelled onwards with 

 greater force. This supplementary propelling force, on the part of the 

 arteries, may serve as a compensation to that diminution of the heart's 

 power, which must result from the increased friction of the blood against 

 the walls of the vessels occasioned by their subdivision ; and we thus 

 observe, even in the highest animals, some traces of that diffused agency, 

 on which the Circulation is so much more dependent in the lower tribes. 



586. It seems probable, however, that one chief use of the Muscu- 

 larity of the Arterial walls, consists in its regulation of the diameter 

 of the tubes, in accordance with the quantity of blood to be conducted 

 through them to any part ; the proper amount being determined by 

 circumstances at the time. Such local changes may form a part of the 

 regular series of actions of the human body, as when the Uterine and 

 Mammary arteries undergo enlargement, at the periods of pregnancy 

 and parturition ; and they occur still more frequently in diseases, which 

 are attended by increased action of particular organs. In such cases, 



