494 OP THE CIRCULATION OP THE BLOOD. 



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 maintenance of the current through them. The 

 influence of the prolonged application of Cold to a part may be quoted in sup- 

 port of the same general proposition ; for, although the caliber of the vessels 

 may be diminished by this agent, yet their contraction is not sufficient to ac- 

 count 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 explained in the succeeding Chapter (Sect. 

 3). At present it may be stated as a fact, which has now been very satisfac- 

 torily ascertained, that, if admission of air into the lungs be prevented, the cir- 

 culation 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 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 As- 

 phyxia in a different manner ; for the same stagnation results, as in the other 

 case. 



526. If the phenomena which have been here brought together be considered 

 as establishing the existence, in all classes of beings possessing a circulating 

 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, what 

 is the nature of that power. It is very doubtful whether the Capillaries possess 

 true contractility ; for, although their diameter is subject 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 alterations in whose caliber under such influences have 

 been already described ( 513). In the experiments of the Profrs. Weber (loc. 

 cit.) the application 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 dilata- 

 tion, or by some kind of peristaltic movement ; and observation completely ne- 

 gatives 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 Capillaries to the movement 

 of blood through them, must be altogether dismissed. There is experimental 

 evidence, however, that the movement of the blood may be affected by any 

 agency which alters the chemico-vital relations between the blood and the tissues 

 which it permeates. Thus, when the interrupted electric current was applied 

 to the capillaries by the Profrs. Weber, they noticed that the blood-corpuscles 

 showed a remarkable tendency to adhere to each other and to the walls of the 



