ill * bd ina. 
it VAS C Um AR SS ¥'s i iM: 
By ALFRED H. YounG and ARTHUR ROBINSON. 
THE vascular system consists of a series of tubular vessels, with more or less 
distinct walls, which run through all parts of the body. Some contain blood, others 
are filled with a colourless fluid called lymph; hence the distinction between the 
blood-vascular system and the lymph-vascular system. The two systems differ, not 
only as regards their contents, but also in their relations to the tissues amongst 
which they he; for whilst the vessels of the former system are closed, those of the 
latter communicate freely with intercellular spaces and serous sacs. 
The blood-vascular system is tubular throughout; the tubes or vessels possess 
distinct walls ; they vary in size and in the structure of their walls, but all con- 
tain blood, which is conv eyed through them to-and from the tissue elements of the 
body. The blood is propelled along the vessels chiefly by a central propulsive 
organ—the heart. The outgoing vessels from the heart, along which blood is 
transmitted to the tissues, are termed arteries : ; the vessels which return blood from 
the tissues to the heart are known as veins; whilst the smallest tubes—those which 
connect the arteries and veins together, constituting at once the terminations of 
the arteries and the commencements of the veims—are called capillaries. 
Blood capillaries are very small (hair-like) vessels with exceedingly thin walls, 
which permit of the easy passage outwards of the nutritive plasma from the blood 
to the tissues, and, in the opposite direction, of some of the products of tissue changes 
as well as of modified food material from the alimentary canal. ; 
Arteries and veins are simply conducting passages; structurally they differ 
from capillaries in the greater complexity of their walls. They vary greatly in size, 
but are always larger than capillaries, and they grow larger and larger the nearer 
they are to the heart. With increase in size there is a corresponding increase in 
the thickness and complexity of their walls. 
Structure of Blood Capillaries. —Capillaries measure from 3,55 to sop of an 
inch in diameter, and about j=. to 34 of an inch in length. Their walls are 
simple, and, in the smallest capillaries, consist principally of elongated elastic endo- 
thelial célls, with sinuous edges, pointed extremities, and oval nuclei. The cells are 
cemented to one another along their margins by intercellular cement, which readily 
stains with nitrate of silver. Here and there the cement substance appears to accu- 
mulate, forming minute spots indicative of the less perfect apposition of the edges 
of the cells. These spots, when small, form the so-called stigmata; when larger 
they are known as stomata. 
The larger capillaries are invested by a connective tissue sheath consisting of 
branched cells which are united together and to the endothelial cells of the capillary 
wall. This sheath is termed the adventitia capillaris. 
Capillaries are arranged in networks, the nature and character of which differ 
in different tissues. The small arteries which end in them are known as capillary 
arterioles, and the venous radicles which commence from them are appropriately 
termed capillary veins. 
Structure of arteries and veins.—The delicate elastic endothehal membrane 
forming the wall of the simplest capillaries extends as a continuous lining through- 
out the whole of the blood-vascular system. The constituent cells are fusiform, 
i 
