MOVEMENT OF THE BLOOD IN THE CAPILLARIES. 349 



to attribute the circulation in the latter to the influence of the heart of the 

 former, propagated through the placental vessels. This supposition has not 

 been disproved (however improbable it might seem), until a case of this 

 kind occurred, which was submitted to the most careful examination by an 

 accomplished anatomist; 1 when the 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 ves- 

 sels. It was a very curious anomaly in this case, that the usual functions 

 of the arteries and veins must have been reversed ; for the Vena Cava, re- 

 ceiving its blood from the umbilical vein nearly as usual, had no communi- 

 cation with the Arterial system (the Heart being absent), except through the 

 systemic capillaries ; to which, therefore, the blood must have next pro- 

 ceeded, 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 every- 

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

 unequivocally demonstrated, furnishes 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 

 maintain 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 

 capability in the living system of adapting itself to exigencies. In the 

 acardiac Fcetus, the " capillary 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 muscular tissue could at last be 

 detected in it, but without any such interruption to the circulation as must 

 have been anticipated, if this organ furnishes the sole impelling force. 



267. Further, it is a general principle, unquestioned by any Physiologist, 

 and embodied in the ancient aphorism, Ubi stimulus, ibi flu.vus, that, when 

 there is any local excitement to the processes of Nutrition^ Secretion, etc., 

 a determination of blood towards the part speedily takes place, and the 

 motion of blood through it is increased in rapidity ; and although it might 

 be urged that this increased determination may not be the effect, but the 

 cause, of the increased local action, such an opinion could not be sustained 

 without many inconsistencies with positive facts. For it is known that such 

 local determinations may take place, not only as a part of the regular phe- 

 nomena of growth and development (as in the case of the entire genital 

 system at the time of puberty and of periodical heat, the uterus after con- 

 ception, and the mamnuB after parturition), but also as a consequence of a 

 strictly local cause. Thus, the student is well aware that, after several 

 hours' close application, there is commonly an increased determination of 

 blood to the brain, causing a sense of oppression, a feeling of heat, and fre- 

 quently a diminished action in other parts; and, again, when the capillary 

 circulation is being examined under the microscope, it is seen to be quick- 

 ened by moderate stimuli, and to be equally retarded by depressing agents. 

 All these facts harmonize completely with the phenomena, which are yet 

 more striking in the lower classes of organized beings, and which are evi- 

 dently in accordance with the same laws. 



1 See Dr. Houston in the Dublin Medical Journal, 1837. An attempt WHS made 

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

 ences ; but a most satisfactory reply was given by Dr. Houston, at the meeting of the 

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

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



