MOTION OF THE BLOOD IN THE CAPILLARIES. 503 



pervious ; so that the cessation of the flow of blood could not be attributed to 

 any impediment, except that arising from the cessation of some power which 

 exists in the capillaries, and which is necessary for the maintenance of the 

 current through them. The influence of the prolonged application of Cold 

 to a part, may be quoted in support of the same general proposition ; for, al- 

 though the calibre of the vessels may be diminished by this agent, yet their 

 contraction is not sufficient to account for that complete cessation of the flow 

 of blood through them which is well known to occur, and to terminate in the 

 loss of their vitality. The most remarkable evidence on this point, however, 

 is derived from the phenomena of Asphyxia, which will be more fully ex- 

 plained in the succeeding Chapter. At present it may be stated as a fact, 

 which has now been very satisfactorily ascertained, that, if admission of air 

 into the lungs be prevented, the circulation through them will be brought to a 

 stand, as soon as the air which they contain has been to a great degree de- 

 prived of its oxygen, or rather has become loaded with carbonic acid; and 

 this stagnation will, of course, be communicated to all the rest of the system. 

 Yet, if it have not continued sufficiently long, to cause the loss of vitality in 

 the nervous centres, the movement may be renewed by the admission of air 

 into the lungs. Now, although it has been asserted that the stagnation is due 

 to a mechanical impediment, resulting from the contracted state of the lungs in 

 such cases, this has been clearly proved not to be the fact, by causing animals 

 to breathe a gas destitute of oxygen, so as to produce Asphyxia in a different 

 manner; the same stagnation results as in the other case. 



739. If the phenomena which have been here brought together, be con- 

 sidered as establishing the existence, in all classes of beings possessing a cir- 

 culating apparatus, of a Capillary power, which affords a necessary condition 

 for the movement of the nutritious fluid, through those parts in which it comes 

 into more immediate relation with the solids, the question still remains 

 open, as to its nature. That the Capillaries possess a contractile power, far 

 higher in degree than that of the large Arteries, and more easily excited than 

 that of the smaller, appears scarcely to admit of doubt; though to what it is 

 due, may be reasonably questioned. It has been recently asserted by 

 Schwann, that they possess the same kind of fibrous tissue in their walls, as 

 do the large vessels: and this cannot be regarded as improbable. It is not 

 possible, however, that their contractility could have any influence in aiding 

 the continuous motion of blood through them ; unless it were exercised in a 

 very different manner from that of which observation affords us evidence. 

 For, when we are microscopically examining the Capillary circulation of any 

 part, it is at once seen, that the vessels present no obvious movement ; and 

 that the stream, now rendered continuous by the elasticity of the arteries, 

 passes through them, as through unelastic tubes. The only method, in which 

 the contractility of the Capillaries could produce a regular influence on the 

 current of blood, would be an alternate contraction and dilatation, or a peri- 

 staltic movement; and of neither of these can the least traces be discerned. 

 Hence we should altogether dismiss from our minds the idea of any 'mechani- 

 cal assistance, afforded by the action of the Capillaries, to the movement of 

 the blood. That the contractile coat of the Capillaries has for its office, to 

 regulate the calibre of the vessels, can scarcely be doubted ; but any general 

 permanent contraction would only occasion an obstacle to the circulation, 

 as is shown by the effects of stimulating injections, which, if thrown into the 

 vessels before their vitality has been lost, will not pass through the capillaries. 

 It would appear, therefore, to be through their action on this coat, that local 

 stimuli occasion a contraction of the capillaries ; their effect, however, is dif- 

 ferent from what might have been anticipated ; for, instead of the capillary 

 circulation being retarded, it is accelerated, at least until an abnormal condition 



