CHAP. XXVIII.] THE CAPILLARY CIRCULATION. 373 



put on record by the late Dr. Houston.* No doubt the same law 

 which influences the movement of fluids in vegetable tissues would 

 be in operation in such cases. 



Now with reference to the phenomena in the circulation of the 

 blood in man above referred to, it may be asked is the assumption 

 of the exercise of a capillary force necessary for explaining them ? 

 Can blushing, and other local determinations of blood be ac- 

 counted for, if we admit a vis a tergo as the sole force of the 

 circulation? In order to explain the accumulation of blood, in 

 the cheeks for instance, under the influence of mental emotion, 

 the advocates of the latter doctrine suppose the capillaries 

 muscular, and affirm that a relaxed state of the walls of the 

 capillary arteries, and perhaps also of the capillaries themselves, is 

 produced by the nervous change which mental emotion excites. 

 Such an explanation is perfectly admissible in this particular case, 

 and it seems highly probable that in the relaxed state of the 

 capillary vessels of the face, their walls yield under the pressure of 

 the heart more than those of neighbouring vessels which do not 

 come so completely within the range of the centre of emotion. 

 And in many persons, emotion causes the blood to desert the cheeks 

 which in consequence become pale. In such cases the change in 

 the nervous centre must excite an opposite, that is, a contracted 

 state of the capillaries of the cheeks. 



But the accumulations of blood which are caused by local irrita- 

 tions, do not admit of satisfactory explanation by mere changes in 

 the capillaries of the affected part. 



For example, a particle of dust is thrown into the eye, and as 

 long as it is in .contact with the conjunctiva, its capillary vessels 

 are turgid with blood. Is this due to a relaxed state of the 

 capillaries caused by the presence of an irritating agent? The 

 analogy of the influence of mechanical stimulation upon other vessels 

 would lead us to infer that the irritation of a particle of dust in 

 contact with the conjunctiva would cause the capillary vessels to 

 contract, and a contracted state of these vessels would oppose rather 

 than favour the accumulation of blood in them. It seems much 

 more reasonable to suppose that the irritation caused by the foreign 

 particle, increases the attractive force which the tissue naturally 

 exercises on the blood ; and this would give us a clue to explain 

 the two kinds of congestion long recognised by practical men, 

 the passive and the active form. The former is owing simply 

 to a relaxed flaccid state of the parietes of the blood-vessels 

 * Dublin Medical Journal, vol. viii. 



