MOTION OF THE BLOOD IN THE CAPILLARIES. 381 



cental vessels. The supposition has not been disproved (however improbable 

 it might seem) until recently ; when a case of this kind occurred, which was 

 submitted to the most careful examination by an accomplished anatomist ;* and 

 this decisive result was obtained, that it seemed impossible for the heart of 

 the twin foetus to have occasioned the movement of blood in the imperfect one ; 

 and that -some cause, present in the latter, must have been sufficient for the 

 propulsion of blood through its vessels. It was a very curious anomaly in this 

 case, that the usual functions of the Arteries and Vpins must have been reversed ; 

 for the Vena Cava, receiving its blood from the Umbilical Vein nearly as usual, 

 had no communication with the Arterial system (the Heart being absent), 

 except through the Systemic Capillaries ; to which, therefore, the blood must 

 have next proceeded, returning to the placenta by the Umbilical Artery. This 

 view of the course of the blood was confirmed by the fact, that the veins were 

 everywhere destitute of valves. It is evident that a single case of this kind, 

 if unequivocally demonstrated, furnished all the proof that can be needed of 

 the existence, even in the highest animals, of a capillary power, which, 

 though usually subordinate to the Heart's action, is sufficiently strong to main- 

 tain the circulation by itself, when the power of the central organ is diminished. 

 In this, as in many other cases, we may observe a remarkable power in the 

 living system, to adapt itself to exigences. In the acardiac Foetus, the capil- 

 lary power supplies the place of the Heart, up to the period of birth ; after 

 which, of course, the circulation ceases, for want of due aeration of the blood. 

 It has occasionally been noticed, that a gradual degeneration in the structure of 

 the Heart has taken place during life, to such an extent that scarcely any mus- 

 cular tissue could at last be detected in it, without any such interruption to 

 the circulation, as must have been anticipated, if it were the sole impelling 

 force. 



508. It is equally capable of proof, on the other hand, that the Capillaries 

 may, by an influence peculiar to them, afford a complete check to the circu- 

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

 mechanical impediment exists to the transmission of blood. Thus, cases of 

 spontaneous gangrene of the lower extremities are of no unfrequent occurrence, 

 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 renewed by the admission of air 

 into the lungs. Now, although it has been asserted, that the stagnation is due 



* See Dr. Houston in the Dublin Medical Journal, 1837. An attempt has been recently 

 made by Dr. M. Hall (Edinb. Monthly Journal, 1843) to disprove Dr. Houston's infer- 

 ences; but a most satisfactory reply has been made by Dr. Houston, at the Meeting of 

 (he British Association, August, 1843, and published in the Dublin Journal, Jan., 1844. 

 See also Edinb. Med. and Surg. Journ., July, 1844. 



