678 SPECIAL PHYSIOLOGY. 



show that this force is really the residue of that of the left ventricle, 

 which is therefore an adequate cause, and probably the true cause of 

 the motion of the blood in the veins, towards the right auricle. First, 

 pressure upon all the arteries of a given part, arrests the flow of blood 

 from a wounded vein belonging to the same part. Secondly, if the 

 venous circulation from a given part, be entirely arrested, by pressure 

 on, or ligature of, the veins, the blood pressure in them is said to rise, 

 so as even to be equal to that in the corresponding arteries (Mageridie). 

 Thirdly, as already stated, when the heart's action is weakened, its 

 rhythmic force is propagated into the capillaries, giving a pulsatory 

 movement to the blood contained in them, and so establishing the fact 

 that the heart's action extends to that part of the circulation ; but, 

 besides this, under certain conditions, oscillations occur in the blood 

 pressure in the veins, as indicated by the hsemadynamometer. Fourthly, 

 water injected into the arteries, with a force less than that of the heart, 

 returns through the veins. Lastly, it has been shown by Dr. Sharpey, 

 that defibrinated ox's blood injected into the thoracic aorta of a dog, 

 passes freely back by the veins of the lower limbs ; also, that if the 

 aorta be tied in the abdomen, below the origin of the arteries of the 

 stomach and intestines, the blood still returns along the inferior vena 

 cava. In the former case, the blood passes through a single capillary 

 system, namely, that of the lower limbs, whilst, in the latter, it is pro- 

 pelled through two sets of capillary vessels, viz., through those of the 

 alimentary canal, into the portal venous system, onwards through the 

 capillary plexuses of the hepatic lobules, and then through the hepatic 

 veins into the vena cava inferior. The pressure employed in these ex- 

 periments, as measured by the haemadynamometer, was maintained at 

 about 6 inches of mercury, which is known to agree with the force of 

 the left ventricle in the living dog. To propel the blood through the 

 pulmonary arteries, capillaries, and veins, a less force was sufficient. 

 From the preceding considerations and experiments, the adequacy of 

 the heart's force, to complete the circulation of the blood back to itself, 

 may therefore be considered as established. The position of this organ 

 in the centre of the circulatory system, its large muscular mass, and 

 the proportionate thickness of the right and left ventricles to the work 

 which each has to perform, likewise favor the conclusion that the heart, 

 when present, is the real agent in the circulation of the blood. A 

 circulation also takes place, however, in so-called acardiac embryos, in 

 which the heart is absent, though in some of these, the movement of 

 the blood may depend on the action of the heart of a conjoined embryo. 

 Again, in the early embryo of the chick, a movement of the blood in 

 the so-called vascular area, is noticeable before the heart begins to pul- 

 sate ; but this movement is irregular, and takes place from the vascular 

 area towards the embryo. Moreover, as we shall hereafter see, a true 

 circulation takes place, in contractile vessels, in certain of the lower 

 animals, which are destitute of a heart. Lastly, in plants, examples 

 are met with of a circulation independent even of contractile vessels 

 or cells. The advocates of the existence of a force, originating in the 

 capillaries or their neighborhood, relying on these and other facts 



