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 vascular system.

(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

ties of each part of the vascular system may be summarized thus:

1. The structure of the walls of the large arteries shows them to be capable of sustaining considerable pressure, and of exerting powerful and continuous elastic recoil on the blood.

2. In the small arteries, as well as this elasticity, frequent variation in their calibre occurs, dependent on the contraction of their muscular coat which regulates the blood flow.

3. In the capillaries we find extreme thinness, elasticity, and permeability of their wall, which presents an immense surface, so as to allow free interchange between the blood and the surrounding textures.

4. The veins have yielding and distensible walls, capacity to accommodate a large quantity of blood, and valves to prevent its backward flow upon the capillaries.

5. The aggregate sectional area of the systemic capillaries is about three hundred times that of the great veins, and seven hundred times that of the aorta, so that the current of the blood must be proportionately slower in the capillary network.

PHYSICAL FORCES OF THE CIRCULATION.

A liquid flows through a tube as the result of a difference of pressure in the different parts of the tube. The liquid moves from the part where the pressure is higher toward that where it is lower, except where sudden and great variations of calibre

The energy of the flow corresponds with the amount of difference in the pressure, and varies in proportion to it, being con

* Although in the whole course of any system of tubes the flow of liquid must take place from the part of higher to that of lower pressure, yet if a narrow tube open abruptly into one the diameter of which for a short length is much greater, the diminution of velocity in the wide tube may cause the local pressure in it to exceed that in the narrower tube immediately preceding; so that the liquid would be actually flowing, for a short distance, from a point of lower to a point of higher pressure.

tinuous so long as the pressure is unequal in different parts, and ceasing when it is equalized throughout the tube.

If liquid be forcibly pumped into one extremity of a long tube, such as a garden hose, a pressure difference is established, the

R.H.

FIG. 129.

Р

L.H.

pressure becoming greater at the end into which the liquid is pumped, a current consequently takes place toward the open end. So long as the free or distal end of the tube is quite open and on the same level as the rest, no very great pressure can be brought to bear on the walls of the tube, no matter how forcibly the pumping may go on, as the liquid easily escapes, and therefore flows the more quickly as the pumping becomes more energetic. If, however, the outflow be impeded by raising the distal end of the tube to any considerable height, or by partially closing the orifice with a nozzle or rose, then the pressure within the tube can be greatly increased by energetic pumping, and the tube being elastic will be distended.

Diagram of Circulation, showing right monary (P) and systemic (s) sets of

(RH) and left (LH) hearts, and the pulcapillaries.

It can be further observed in this common operation that the smaller the orifice of the nozzle the greater the pressure in the tube with a given rate of working the pump; and, the orifice remaining the same, the pressure will increase in proportion as the pump is more energetically worked. Or in other words, the pressure within the tube will depend on (a) the energy used at the pump, and (b) the degree of impediment offered to the outflow.

If the tube be resilient, and the nozzle have a small orifice so that a high pressure can be established within the tube, it will be found that the liquid will flow from the nozzle in a continuous stream, and will not follow the jerks communicated by the pump. That is to say, the interrupted energy of the pump is stored up by the elastic tube and converted into a continuous pressure exerted on the fluid. But if the tube be quite rigid, or the orifice too wide to allow the pressure within the tube to be raised