350 OF THE CIRCULATION OF THE BLOOD. 



268. It is equally capable of proof, on the other hand, that au influence 

 generated in the Capillaries may afford a complete check to the circulatioii 

 in the part; even when the Heart's action is unimpaired, and no mechanical 

 impediment exists to the transmission of blood. Examples of this may be 

 seen in the loss of vitality produced by the prolonged application of cold to 

 a part; also in cases of spontaneous gangrene of the lower extremities, in 

 which the death of the solid tissues is clearly connected with a local decline 

 of the circulation, and in which it has been shown by examination of the 

 limb after its removal, that both the larger tubes and the Capillaries were 

 completely 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 main- 

 tenance of the current through them. The most remarkable evidence on 

 this point, however, is derived from the phenomena of Asphyxia, which will 

 be more fully explained 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 deprived 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 re- 

 newed by the admission of air in the lungs. Now although it has been as- 

 serted, 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; for the same stagnation 

 results as in the other case. 



209. 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 con- 

 dition 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, what is the nature of that power? It is very doubtful whether 

 the Capillaries possess true contractility; for although their diameter is sub- 

 ject to great variation, yet this may be due simply to the elasticity of their 

 walls, which tends to keep them constantly contracted upon the stream of 

 blood that passes through them ; and there is no adequate proof that the 

 alterations in their size, which are consequent upon the local application of 

 stimuli, proceed from any other source than the alteration in the quantity of 

 blood delivered to them by the minute arteries, the very considerable altera- 

 tions in whose calibre under such influences have been already described 

 ($ 252, 253). In the experiments of the Profs. Weber (loc. fit.), the ap- 

 plication of the electric stimulus to the Capillaries produced no change in 

 their diameter. Even supposing the Capillaries, however, to possess such 

 an independent contractility, this could not exert itself in aiding the flow of 

 blood through them, except either by rhythmical alternations of contraction 

 and dilatation, or by some kind of peristaltic movement; and observation 

 completely negatives the idea of the existence of any such movement, since 

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

 passes through the Capillaries as through tubes of glass. Hence the notion 

 of any mechanical assistance, afforded by the action of the walls of the Cap- 

 illaries to the movement of blood through them, must be altogether dis- 

 missed. 



270. There is experimental evidence, however, that the movement of the 



