MOTION OF THE BLOOD IN THE CAPILLARIES. 383 



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 different from 

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

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

 from their continued operation. Here, again, is another evidence, that some- 

 thing different from mechanical power must be the agent that operates in all 

 the foregoing cases. 



511. The nature of this agent is at present very obscure ; and it may- not 

 be in our power for some time to unveil it. The conditions of its action, how- 

 ever, lie open for investigation ; and it appears from the foregoing facts that a 

 very simple and constant expression of these may be given. Whilst the 

 injection of blood into the capillary vessels of every part of the system is due 

 to the action of the heart, its, rate of passage through those vessels is greatly 

 modified by the degree of activity in the processes, to which it should normally 

 be subservient in them ; the current being rendered more rapid by an increase 

 in their activity, and being stagnated by their depression or total cessation. 

 Thus it seems that "the capillaries possess a distributive power over the blood, 

 regulating the local circulation independently of the central organ, in obedience 

 to the necessities of each part."* If this be true, it is evident that the dilata- 

 tion or contraction of the capillaries will only have a secondary influence on 

 the movement of the blood through them. The former condition is usually 

 an indication of diminished vital energy ; and when it is observed, it is almost 

 invariably accompanied by a retardation or partial stagnation of the current ; 

 on the other hand, the application of a moderate stimulus, which excites the 

 contractility, accelerates for a time the motion of the blood, by rendering more 

 energetic that reaction between the fluids and the surrounding tissues, which 



^)is the condition that really has the most influence over the current. It is not 

 ^enough to object to such a doctrine, that we know nothing of the mode in which 

 i this reaction affects the movement of the blood ; since we are equally ignorant 

 f\)f the modus operandi of many other causes whose real existence is fully 

 Acknowledged, as, for instance, the effect of a stimulus applied to a motor 

 j*Yierve, in causing contraction of the muscle supplied by it. 



512. An attempt has been made by Dr. Alison, to give more precision to 

 foregoing statement, by attributing the effect to a series of " vital attrac- 

 s and repulsions," created by the operations to which the blood in the 



pillaries is subservient. He considers that the particles of blood are drawn 

 irds the solids surrounding the capillaries, so long as they have not come 

 close relation with them ; but that, after accomplishing the purposes of 

 circulation, they are again repelled by the same property. It is very 

 possible that these attractions and repulsions may have a real existence, and 

 may be the operative causes in producing the phenomena in question, without 

 being essentially different in character from those which are witnessed in physics 

 and chemistry : it seems desirable, therefore, not to apply to them the term vital, 

 which denotes, if it mean any thing, that they are to be referred to a set of laws 

 entirely distinct. That alterations in the chemical state of the blood (involving, 

 of course, important changes in its vital properties), are capable of exercising 

 a most important effect on the capillary circulation, is shown not merely by the 

 phenomena of Asphyxia, already referred to, but by the curious fact recently 

 ascertained by Dr. J. Reid, that the blood, when imperfectly arterialized, is 



* See Palmer's edition of Hunter, vol. iii., p. 232. Note by Mr. P. 



