672 CIRCULATION. 



the vessels. 2. The experiments of Hales and accompanies these partial distributions of 



Wedemeyer shewing that, according to the blood to particular parts, yet they all suffi- 



more or less stimulating character of the fluids, ciently demonstrate that while the heart's action 



their passage through the vessels was more or remains the same, the quantity of blood sent to 



less eas>y. 3. The experiments shewing that, particular parts must have been modified by 



in an animal which has been dead for some some action of the vessels themselves, 



time, steeping of the body in warm water, and There are some physiologists, however, who 



the injection of warm water into the vessels, hold the opinion that the motion of the blood 



so as to clear the passage through them, puts is promoted in some way or other (they do not 



the vessels in such a condition that a force of a sufficiently clearly explain how) by powers 



few pounds is sufficient to effect the pro- acting on it during its passage through the 



pulsion of fluids through them. 4. The ob- capillary vessels; and there are a few who 



servations of Haller, Spallanzani, Magendie, have gone so far as to suppose that the 



and others, that all regular progressive motion heart drives the blood only as far as the capil- 



of blood in a vein, or the issue of blood from laries, from whence it is propelled onwards 



an orifice in a vein, ceases very soon after into the veins by powers originating in the 



the heart's action is suspended, or when any small vessels themselves. These opinions have 



obstacle prevents its force being communicated been supported chiefly by arguments drawn 



to the blood in the veins. 5. The observations from the facts already mentioned as illustrating 



of Spallanzani, Thomson, and others, that the the power of the small vessels to modify the 



impulses of the heart are visibly continued on circulation or to cause local variations in the 



through the small arteries and capillaries, and distribution of the blood, as also on the fol- 



even into the veins in some states of the circu- lowing grounds, which are ably stated in a 



lation. This phenomenon is most apparent at supplement to his Outlines of Physiology,* 



the time when the action of the heart is weak, recently published by Professor Alison, of 



and in such states of the circulation this re- Edinburgh, who is one of those who have 



mittent flow of the blood may be converted more lately adopted this opinion, and by Dr. 



into a merely oscillatory movement without Black in an ingenious essay on the capillary 



any regular progression by the gradual increase circulation .f 



of the pressure applied to the artery which Besides the analogical argument drawn from 



supplies the blood to the capillary vessels the lower animals having a circulation of fluids 



under observation; a fact which shews dis- without any heart, and the supposed unaided 



tinctly on the one hand that the force of the circulation in acardiac foetuses, it is stated 



heart is continued on through the capillaries, that 



and on the other that when a resistance is op- 1. After the heart of the frog or such cold- 

 posed to the progress of the action of the blooded animals has been cut out, or a liga- 

 heart through the arteries, no other force then ture passed round the aorta, some motion of 

 operates sufficient to cause a continued and the blood still continues to occur for a few 

 progressive motion of the blood. minutes in the small vessels ; and it is farther 

 But, although the small vessels do not con- stated, that this motion is influenced by heat, 

 tribute by their active contraction to propel the by certain applications to the web of the frog's 

 blood through them, or although they do not foot, and the state of the nervous system .J 

 as a whole assist the force of the heart, it is 2. That while the circulation is going on 

 yet very apparent that they have the power of with its usual freedom, the direction and velo- 

 modifying in a remarkable manner the flow of city of the flow of blood are subject to sud- 

 blood in particular parts. Among the circum- den or rapid changes which do not admit of 

 stances which prove this power of the small being accounted for simply by contractions of 

 vessels to modify the circulation may be men- the vessels. 



tioned the various instances in which there 3. That the blood when out of the vessels, 



occur local determinations to particular parts, immediately after it has been drawn, or when 



unaccompanied by any change in the action of extravasated in the textures, performs motions 



the heart or in the general circulation. 1. The which seem to belong to itself or are spon- 



act of blushing and erection, or the reverse taneous. 



actions of paleness, collapse, &c. which seem 4. That the passage of the blood through 



to depend, in most instances at least, on some the capillary vessels of the lungs is imme- 



change in the terminal vessels 2. Inflam- diately influenced by the chemical change of 



mations or hemorrhages confined to a parti- the venous blood into arterial, for its velocity 



cular part of the body. 3. The increase or is diminished as soon as this change does not 



decrease of secretions from glands, periodical occur. || 



or instantaneous. 4. The increased size of 5. That the remoteness of the capillaries of 



the vessels of the uterus during pregnancy, the vena portae of the liver from the heart ren- 

 of the mammae after child-birth, &c. 5. The 



enlargement of bloodvessels in new growths, * Outlines of Physiology, Supplement to 2d 



tumours, &c. 6. The enlargement of collateral edit. Edin. 1836. 



anastomosing vessels, after the closure of the t London, 1825. 



principal trunk of a limb. And, 7. The unequal * Caller, G"Uot, Leuret and Wilson 



, c j-rc f.-> Marshall Hall, and others. 



growth or development of different parts of the $ Kielmeyer, Treviranus, Cams, Czermack, 



foetus. Although we do not understand the (Esterreicher, and Schultz. 



nature of the change in the vessels which || Dr. Alison, loc. cit. 



