562 OF THE CIRCULATION OF BLOOD. 



impossible for the heart of the twin fetus 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 Capil- 

 laries ; 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, 

 furnishes all the proof that can be needed, of the existence, even in the high- 

 est 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 power in the living system, to adapt itself to 

 exigencies. In the acardiac F(etus, the capillary power supplies the place 

 of the Heart, up to the period of birth ; after which, of course, the circula- 

 tion 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 ; without any such interruption to the circulation, as 

 must have been anticipated, if it furnished the sole impelling force. 



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

 and embodied in the ancient aphorism Ubi stimulus, ibi jluxiis, that, when 

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

 nations 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 mammee 

 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 equally re- 

 tarded 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 the results of the same laws. 



738. It is equally capable of proof, on the other hand, that an influence 

 generated 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 

 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 



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

 Association, August IS-l.'i, and piibli>licd in the Dublin Journal, Jan. 1844. See also Edinb. 

 Med. and Surg. Joiirn. July 1S44. 



