218 ON THE EARLY STAGES OF INFLAMMATION 
SECTION II 
On the Structure and Functions of the Blood-vessels 
An acquaintance with the anatomy and physiology of the vascular system 
is indispensable to a successful study of the deviations from health exhibited 
in the circulation of the blood through the vessels of an inflamed part; it is 
not, however, intended to give here a full account of the subject, but merely to 
dwell upon some important points on which differences of opinion prevail. 
It has long been a debated question whether or not the capillaries possess 
contractility, and there is still some difference of opinion among authorities 
upon the subject. With a view to throwing light upon this important point, 
I investigated carefully the structure of the minute vessels of the frog’s foot ; 
dissecting them out from between the layers of skin composing the web, so as to 
render their constituent material capable of clear definition with the microscope. 
The chief results have been communicated to the Royal Society of Edinburgh, in 
a paper that will shortly appear in their Tvansactions, ‘On the Structure of 
Involuntary Muscular Fibre.’* I need therefore merely repeat here, that while 
the capillaries were proved to consist, as has been long known, merely of a delicate 
homogeneous membrane beset with occasional nuclei, the minute arteries, some 
of them even less in calibre than average capillaries, were found to possess three 
distinct coats, namely, an external layer of cellular tissue, in variable quantity, 
longitudinally arranged, an internal extremely delicate lining membrane, and 
an intermediate circular coat, which constituted the principal bulk of the vascular 
parietes, and which, when highly magnified, was found to consist of a single 
layer of muscular fibre-cells, each wound spirally round the internal membrane 
so as to encircle it from one and a half to two and a half times. 
Now when we consider the properties of muscular fibre-cells, which, as is 
shown in the paper referred to, are capable of contracting in the pig’s intestine 
as much as to one-tenth of their length, it is impossible to conceive a more efficient 
mechanism for the constriction of a tube than is provided in these minute 
arteries. On the other hand, the capillaries are totally destitute of any structure 
known to be contractile. The changes of calibre which occur in the vessels 
of the living web are in perfect harmony with this anatomical description ; for 
while the arteries, even to their smallest branches, are sometimes constricted 
to absolute closure, and at other times widely dilated, the capillaries are never 
found to be entirely closed, nor to present any variations in diameter, which are 
not explicable by elasticity of their parietes.’ 
* Printed at p. 15 of this volume. 
* In this respect I merely confirm the observations long since made by Messrs. Pagetand Wharton Jones. 
