222 ON THE EARLY STAGES OF INFLAMMATION 
The veins of the frog’s web afford very little evidence of contractility ; 
but a small amount of unstriped muscular tissue, transversely arranged, is 
distinctly to be seen in the larger venous branches; and on one occasion I 
observed a very considerable degree of local contraction, as measured from the 
outer borders of the external coat of a vein running through a small area which 
I had pinched forcibly with forceps. I have also seen one expand on sudden 
dilatation of the arteries of the web, so that its diameter increased from twelve 
to fourteen degrees of a micrometer ; but this is perhaps explicable by elasticity. 
It has been already mentioned that the arteries undergo spontaneous 
variations of calibre. Such changes are constantly going on at varying intervals, 
there being nothing of a rhythmical character about them. A struggle on the 
part of the animal is generally accompanied by a very considerable constriction 
of the arteries, and sometimes by absolute closure of them. The contraction 
usually begins a very short time before the motions of the body, so that the 
struggle can commonly be predicted by the appearance of the vessels; and 
dilatation occurs when the creature becomes quiet. Hence the changes of 
calibre are evidently under the control of the nerves. An account of an inquiry 
into the parts of the nervous system by which this control is exercised, will be 
found at p. 27 of this volume; and from the experiments there recorded, it 
will be seen that either extreme constriction or full dilatation of the arteries 
of the web may be induced at pleasure, by operating upon the spinal cord. 
A very good opportunity is thus afforded for studying the effects produced upon 
the capillary circulation by changes of calibre in the arteries, without employing 
any means acting directly upon the foot. This is a matter of very great impor- 
tance, for applications made to the web for the purpose of inducing alterations 
in the dimensions of the vessels, give rise at the same time to other consequences 
of irritation, which complicate such experiments in a most deceptive manner, 
so as to have misled, as I believe, some of the best observers who have devoted 
attention to this subject. 
The following account embodies the results of numerous observations in 
* Since the reading of this paper I have noticed striking examples of the contractility of the larger 
veins in the higher animals. Thus, on exposing the jugular in a living calf, I have seen a particular 
part of the vessel irritated by the process of dissection shrunk to about a third of its previous calibre. 
In the human subject, too, when amputating lately at the shoulder-joint on account of contusion in- 
flicted by machinery upon a previously healthy limb, I noticed the axillary vein reduced to about half 
its natural calibre at the part where it was divided, which was in the immediate vicinity of the injury. 
I have also had occasion to observe the post mortem contractions of the subcutaneous veins of the sheep’s 
foot, which are carried to such an extent as to reduce the vessels from the size of a crowquil to about 
that of a darning-needle. The minute veins also sometimes exhibit great contractility in the higher 
animals, as in the irregular constrictions often seen in those of the mesentery of the mouse, and in the 
remarkable rhythmical variations in calibre discovered by Mr. Wharton Jones in those of the bat’s 
wing (Philosophical Transactions, 1852). 
