672 



CIRCULATION. 



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 i5 op- 



osed to the progress of the action of the 

 eart through the arteries, no other force then 

 operates sufficient to cause a continued and 

 progressive 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 mammae after child-birth, &c. 5. The 

 enlargement of bloodvessels in new growths, 

 tumours, Sec. 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 

 particular 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/}- 



Besides the analogical argument drawn from 

 the lower animals having a circulation of fluids 

 without any heart, and the supposed unaided 

 circulation in acardiac fetuses, 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 of the nervous system. J 



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. 1 1 



5. That the remoteness of the capillaries of 

 the vena portae of the liver from the heart ren- 



* Outlines of Physiology, Supplement to 2d 

 edit. Edin. 1836. 



t London, 1825. 



J Haller, Guillot, Leuret and Wilson Philip, 

 Marshall Hall, and others. 



$ Kielmever, Treviranus, Cams, Czermack, 

 tEsterreicher, and Schultz. 



|| Dr. Alisou, loc. cit. 



