and unstriated muscle.

It is much thicker in the arteries than

in the veins, where its special functions are not required. It

Transverse Section of part of the Wall of the Posterior Tibial Artery (man). (Schäfer.) (a) Endothelium lining the vessel, appearing thicker than natural from the contraction of the outer coats.

(8) The elastic layer of the intima.

(c) Middle coat composed of muscle fibres and elastic tissue.

(d) Outer coat consisting chiefly of white fibrous tissue.

differs somewhat in character in arteries of different calibre, being much thicker in the large vessels. This change occurs

FIG. 125.

N

gradually on passing along the diminishing branches. In the large arteries and the arterioles the middle coat differs essentially both in structure and in function, and in each class of vessel it forms the most important part for the due performance of their respective functions. In the large vessels it is M made up of fibres and sheets of elastic tissue woven into a dense feltwork, interspersed with a

Portion of Small Artery from Submucous few muscle cells. In the smallest Tissue of Mouse's Stomach, stained with

gold chloride, showing the nuclei of the arteries or arterioles, on the muscle cells (M) passing transversely

around the vessel to form the middle other hand, the great mass of the coat, outside which is the fibrous tissue

of the outer coat (F).

several fine nerve fibrils form a network (N).

muscle cells, the elastic tissue being but sparsely represented.

Between the large arteries and the capillaries every grade of

transition may be found; the elastic tissue gradually becoming less abundant and the muscle elements relatively more numerous in proportion as the capillaries are approached.

3. The internal lining (tunica intima) of the arteries is composed of a delicate, elastic, homogeneous membrane lined with a single layer of endothelial cells. The intima may be said to be continuous throughout all the vessels and the heart cavities.

It is thus seen that the large arteries have extremely elastic

and firm walls, capable of sustaining considerable pressure. The smaller the calibre of the arteries becomes the more the general property of elasticity and resiliency is reinforced by that of vital contractility due to the greater relative number of muscle cells contained in the middle coat.

CAPILLARIES.

The frequently branching arterioles finally terminate in the capillaries, in which distinct branches can no longer be recog

nized, but the thin canals are interwoven into a network of blood channels, the meshes of which are made up of vessels, all of which have about the same calibre. They communicate indefinitely with the capillary meshworks of the neighboring arterioles, so that any given capillary area appears to be one continuous network of tubules, connected here and there with the similar networks from distinct arterioles, and thus any given capillary area may be fed with blood from several different sources. The walls of the capillaries are composed of a single layer of elongated endothelial cells (possibly lining an invisible membrane) cemented edge to edge to form a tube. They are

soft and elastic, and permeable not only to the fluid portion of the blood, but also, under certain circumstances, to the corpuscles.

It is, in fact, in these networks that the essential function of the circulation is carried on, viz., the establishment of a free interchange between the tissues and the blood.

The characters of the capillary network vary in the different tissues and organs; the closeness and wideness of the meshes may be said to be in proportion to the functional activity or inactivity of the organ or tissue in question, a greater amount of blood being required in the parts where energetic duties are performed.

VEINS.

The veins arise from the capillary network, commencing as radicles which unite in a way corresponding to the division of the arterioles, but they form wider and more numerous channels. They rapidly congregate together to make comparatively large vessels, which frequently intercommunicate and form coarse and irregular plexuses. The general arrangement of the structures in the walls of the veins is like that of the arteries; they also have three coats, the external, middle and internal; the tissues of each differing but little from those of the arteries. The external coat is like that of the arteries, but is not quite so strong. The middle coat, however, in the large veins, is easily distinguished from that of the large arteries by being much thinner, owing to the paucity of yellow elastic tissue. It is also characterized by its relative richness in muscle fibre. The structure of the middle coat of the small veins can be distinguished from that of the arterioles by the comparative sparseness of the muscle cells running around the tubes. The inner coat of the veins is practically the same as that of the arteries.

The veins are capable of considerable distention, but, though possessed of a certain degree of elasticity, they are much inferior to the arteries in resiliency.

In a large proportion of veins, valve-like folds of their lining coat exist, which prevent the backward flow of blood to the capillaries and insure its passage toward the heart. These valves resemble in their general plan the pocket valves that protect the great arterial orifices of the heart. They vary much in arrangement, there being commonly two or sometimes only one flap or pocket entering into the formation of the valve. They are closely set in the long veins of the extremities, in which the blood current has to move against the force of gravity.

AGGREGATE SECTIONAL AREA OF THE VESSELS.

The general aggregate diameter of the different parts of the vascular system varies greatly. The combined calibre of the branches of an artery exceeds that of the parent trunk, so that the aggregate sectional area of the arterial tree increases as one

proceeds from the aorta toward the capillaries. After the muscular arterioles are passed the general diameter of the vascular system suddenly increases immensely, and in the capillaries it reaches its maximum, the aggregate sectional area of which is said to be several (5 to 8) hundred times as great as that of the aorta.

The aggregate sectional area of the veins diminishes as the tributaries unite to form main trunks, and reaches its minimum at the entrance of the vena cava into the right auricle.

Diagram intended to give an idea of the aggregate sectional area of the different parts of the

(A) Aorta.

(V) Veins.

The transverse measurement of the shaded part may be taken as the width of the various kinds of vessels, supposing them fused together.

The capacity of the veins is, however, everywhere much greater than that of the corresponding arteries, the least difference being near the heart, where the calibre of the vena cava is more than twice that of the aorta.

After this brief anatomical sketch, the most important proper