The principal reasons which we feel inclined to adduce for believing that the heart's action is continued onwards through the capillaries, and_ is sufficient to return the blood through the veins back as far as the heart itself, are the following That in an animal recently killed a very small force only is requisite to cause bland fluids to follow the course of the blood, provided the injection be made before the tonic contraction lia,s had time.to constrict the vessels. 2. The experiments of Hales and Wedemeyer shewing that, according to the more or less stimulating character of the fluids, their passage through the vessels was more or less easy. 3. The experiments shewing that, in an animal which has been dead for some time, steeping of the body in warm water, and the injection of warm water into the vessels, so as to Clear the passage through them, puts the vessels in such a condition that a force of a few pounds is sufficient to effect the pro pulsion of fluids through them. 4. The ob servations of Haller, Spallanzani, Magendie, and others, that all regular progressive motion of blood in a vein, or the issue of blood from an orifice in a vein, ceases very soon after the heart's action is suspended, or when any obstacle prevents its force being communicated to the blood in the veins. 5. The observations of Spallanzani, Thomson, and others, that the impulses of the heart are visibly continued on through the small arteries and capillaries, and even into the veins in some states of the circu lation. This phenomenon is most apparent at the time when the action of the heart is weak, and in such states of the circulation this re mittent flow of the blood may be converted into a merely oscillatory movement without any regular progression by the gradual increase of the pressure applied to the artery which supplies the blood to the capillary vessels under observation ; a fact which shews dis tinctly on the one hand that the force of the heart is continued on through the capillaries, and on the other that when a resistance i3 op posed to the progress of the action of the heart through the arteries, no other force then operates sufficient to cause a continued and ptogressive motion of the blood.
But, although the small vessels do not con tribute by their active contraction to propel the blood through them, or although they do not as a whole assist the force of the heart, it is yet very apparent that they have the power of modifying in a remarkable manner the flow of blood in particular parts. Among the circum stances which prove this power of the small vessels to modify the circulation may be men tioned the various instances in which there occur local determinations to particular parts, unaccompanied by any change in the action of the heart or in the general circulation. 1. The act of blushing and erection, or the reverse actions of paleness, collapse, &c. which seem to depend, in most instances at least, on some change in the terminal vessels 2. Inflam mations or hemorrhages confined to a parti cular part of the body. 3. The increase or decrease of secretions from glands, periodical or instantaneous. 4. The increased size of
the vessels of the uterus during pregnancy, of the mammm after child-birth, &c. 5. The enlargement of bloodvessels in new growths, tumours, &c. 6. The enlargement of collateral anastomosing vessels, after the closure of the principal trunk of a limb. And, 7. The unequal growth or development of different parts of the foetus. Although we do not understand the nature of the change in the vessels which accompanies these partial distributions of blood to particular parts, yet they all suffi ciently demonstrate that while the heart's action remains the same, the quantity of blood sent to paiticular parts must have been modified by some action of the vessels themselves.
There are some physiologists, however, who hold the opinion that the motion of the blood. is promoted in some way or other (they do not sufficiently clearly explain how) by powers acting on it during its passage through the capillary vessels; -and there are a few who have gone so far as to suppose that the heart drives the blood only as far as the capil laries, from whence it is propelled onwards into the veins by powers originating in the small vessels themselves. These opinions have been supported chiefly by arguments drawn from the facts already mentioned as illustrating the power of the small vessels to modify the circulation or to cause local variations in the distribution of the blood, as also on the fol lowing grounds, which are ably stated in a supplement to his Outlines of Physiology,"' recently published by Professor Alison, of Edinburgh, who is one of those who have more lately adopted this opinion, and by Dr. Black in an ingenious essay on the capillary circulation.t Besides the analogical argument drawn from the lower animals having a circulation of fluids without any heart, and the supposed unaided circulation in acardiac fcetuses, it is stated that 1. After the heart of the frog or such cold blooded animals has been cut out, or a liga ture passed round the aorta, some motion of the blood still continues to occur for a few minutes in the small vessels ; and it is farther stated, that this motion is influenced by heat, by certain applications to the web of the frog's foot, and the state bf the nervous system4 2. That while the circulation is going on with its usual freedom, the direction and velo city of the flow of blood are subject to sud den or rapid changes which do not admit of being accounted for simply by contractions of the vessels.
3. That the blood when out of the vessels, immediately after it has been drawn, or when extravasated in the textures, performs motions which seem to belong to itself or are spon taneous.§ 4. That the passage of the blood through the capillary vessels of the lungs is imme diately influenced by the chemical change of the venous blood into arterial, for its velocity is diminished as soon as this change does not occur.II 5. That the remoteness of the capillaries of the vena portm of the liver from the heart ren ders probable the existence in them of some power capable of propelling the blood inde pendently of the heart's action.