CHAP. XXVIII.] CIRCULATION IN THE VEINS. 375 



in points of it, here and there, determining an increased flow of 

 blood to this point and that, and leaving the intervening parts un- 

 affected. The well-ascertained fact, ubi stimulus, ibi fluxus, cannot 

 be so well explained on the hypothesis that the stimulus creates 

 a relaxation of the tunics of the capillaries of the part, because that 

 is opposed to analogy, as on the supposition that under the operation 

 of the stimulus the demand for blood in the tissues is augmented, 

 and the capillary force becomes exalted in the part, in virtue of 

 which a greater flow of blood is determined to it. 



The phenomena of asphyxia show that a stoppage of the circula- 

 tion may take place at the capillaries, notwithstanding the con- 

 tinuance of the heart's action. When the access of air to the lungs 

 is excluded, the circulation ceases at the pulmonary capillaries, and 

 on examination after death the left auricle and ventricle are found 

 quite empty, and the right cavities of the heart gorged with blood. 

 The repletion of the latter cavities, and the emptiness of the former, 

 indicate the position at which the obstruction to the circulation 

 took place. Instantly the air is re-admitted to the lungs, the blood 

 assumes a bright red colour and the circulation goes on, indicating 

 that the changes which take place between the air and the blood, 

 must generate a force which exercises an important influence on 

 the capillary circulation, and without which the heart's force is in- 

 sufficient to propel the blood through the pulmonary vessels. Dr. 

 John Reid* argues, as we think with justice, from the instanta- 

 neousness of the restoration of the circulation on the re-admission of 

 air, that its stoppage must be due to the cessation of the respiratory 

 changes, and not to a contracted state of the capillary arteries as 

 suggested by Mr. Erichsen, because the relaxation of those arteries, 

 like their contraction, is a slow process, requiring two or three 

 minutes to accomplish it. 



* John Reid, on the Cessation of tho Vital Changes in Asphyxia. Phys. Re- 

 searches, Ettinb. 1848. 



i Our friend and colleague, Dr. George Johnson, has discovered a very in- 

 teresting point, connected with the minute vessels of the kidney in cases of 

 chronic nephritis with shrinking of the organ, which furnishes an additional 

 instance of retardation or stoppage of the circulation at the capillaries, despite 

 the continuance of the heart's action. Dr. Johnson shews that, under the in- 

 fluence of defective secretion, the renal circulation is greatly retarded in the 

 intertubular capillaries, and that the Malpighian capillaries are consequently 

 subjected to a greatly increased pressure and distension, giving rise to an 

 escape of serum, or of blood when a rupture of one or more of the minute 

 vessels has occurred. When such a state of vessels has been of long duration, 

 as in chronic inflammation of the kidneys, Dr. Johnson finds the capillary 

 arteries much thickened by reason of hypertrophy of their circular and longi- 



