MOTION OF THE BLOOD IN THE CAPILLARIES. 493 



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 veins 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 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 j" 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 Foetus, 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 inter- 

 ruption to the circulation as must have been anticipated, if this organ furnished 

 the sole impelling force. 



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

 and embodied in the ancient aphorism Ubi stimulus ibifluxm, that, when there 

 is any local excitement to the processes of Nutrition, Secretion, &c., a determi- 

 nation of blood towards the part speedily takes place, and the motion of blood 

 through it> is increased in rapidity j 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 phenomena 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 conception, and the mammae 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 frequently a diminished action in other parts ; and, again, when the 

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

 quickened 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 evidently in 

 accordance with the same laws. 



525. It is equally capable of proof, on the other hand, that an influence gene- 

 rated in the Capillaries may afford a complete check to the circulation in the 

 part; even when the Heart's action is unimpaired, and no mechanical impedi- 

 ment 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 



1 See Dr. Houston in "the Dublin Medical Journal," 1837. An attempt was made by 

 Dr. M. Hall ("Edinb. Monthly Journal," 1843) to disprove Dr. Houston's inferences; but 

 a most satisfactory reply was given by Dr. Houston, at the Meeting of the British Asso- 

 ciation, August, 1843, and published in the "Dublin Journal," Jan., 1844. See also 

 "Edinb. Med. and Surg. Journ.," July, 1844. 



