CIRCULATION. 



terminations of that fluid, which take place in 

 blushing, inflammation, and other states of the 

 economy in which particular parts of the vas- 

 cular system become more or less filled with 

 blood than usual ; for such variations in the 

 distribution of the blood would be impossible, 

 were an alteration in the powers of the heart 

 alone the only means of modifying the circula- 

 tion. The questions, however, whether such 

 powers as are possessed by the arteries contri- 

 bute upon the whole to the progressive motion 

 of the blood or modify only its distribution, 

 are quite distinct from one another. 



In its anatomical structure the fibrous coat 

 of the arteries differs considerably from muscu- 

 lar substance, and appears to resemble more 

 nearly the yellow elastic ligamentous tissue. 

 Its fibres are less mixed with cellular substance 

 than those of muscles ; they are also more dry, 

 hard, and friable, less coloured, and, accord- 

 ing to Hodgkin and Lister,* are destitute of 

 those transverse stria? or lines observed by the 

 microscope in ordinary muscular fibres. The 

 chemical constitution of the middle coat of the 

 arteries differs also from that of muscle, for it 

 is less soluble in acetic acid, and more easily 

 so in mineral acids, and it is believed by Ber- 

 zelius and Young not to contain the animal 

 principle, fibrine, peculiar to muscular flesh. 

 Although we fully admit the importance of 

 these observations as establishing anatomical 

 and chemical distinctions between muscular 

 substance and the texture of the middle coat of 

 the arteries, they do not appear to us to warrant 

 the conclusion too hastily deduced from them 

 by some, that this coat cannot be irritable, or 

 does not possess any of the same properties as 

 muscle, the existence or non-existence of which 

 must be ascertained principally by physiologi- 

 cal evidence. For the transverse stria; cannot 

 be considered as characteristic of all muscular 

 fibres; and were we to reason in this way 

 from the result of anatomical observations 

 only, we should be necessitated to deny the 

 irritability of various other textures, the con- 

 tractility of which from stimulation or without 

 it, is universally admitted, although anatomists 

 have not yet detected muscular fibres in them. 



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 



* Appendix to the Transl. of Edwards 's Work 

 on the Influence of Physical Agents, &c. p. 443. 

 VOL. I. 



it seems by some to have been inferred from 

 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,f Dr. Hastings,! 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 Hales|| 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. 



Haller, though considering the middle coat 



* Lect. on Inflammation. Edin. 1813. 



t Introduct. to the second part of his work on 

 Ftver. 



i Introduct. to his work on the Inflammation of 

 the Mucous Membrane, &c. 



Exper. circa statum Sang, et Vasov. in Inflam- 

 matione. Munich, 1826. 



|| Unteisuch. iiber den Kreislauf dcs Blutes, &c. 

 Hannover, 1828. See also KochinMeckel's Archiv. 

 1832, p. 121. 



H Statical Essays, ii. p. 124. 



2 x 



