not penetrate their ultimate elements; in muscle, for example, they pass between the fibres, external to the

Diagram to illustrate the Tubular Spaces which, according to Wedemeyer, unite the terminal arteries and veins.

sarcolemma. Their distribution follows the anatomical arrangement of the various tissues, and when those of the muscle, the skin, stomach membrane, villi, &c., are injected, they are easily distinguished, and form some of the most brilliant of microscopic objects. On the surface of the body their distribution is very unequal, a cir

cumstance which but serves to enhance by contrast the beauty of the fair bosom and of the blushing cheek.

Capillaries are intermediate between arteries and veins, save in the erectile tissues, uterus, and in the bat's wing, according to Paget a statement, however, denied by Wharton Jones, another most accurate observer.

The microscope, when any transparent part-as the web of frog's foot-is placed within its field, displays the

capillary circulation, a sight full of wonder and interest. It is here roughly sketched the arrows distinguishing the artery and vein; and in the next woodcut a vein is perceived lying out of focus. Fancy a map, with all its rivers and their minutest tributaries to be set flowing. The larger capil

Artery, Vein, and Capillaries of Web laries admit 3 or 4 bloodcells abreast-the smaller

of Frog's Foot.

but 1, which has even to squeeze and elongate before it can pass to a large vessel, when it becomes circular again. The condition of

water in rivers is analogous to that of blood in capillaries, for the current through the centre is far more rapid than that at the sides. In this "still layer" the white cells lie, apparently ministering to nutrition. They keep so distinct that it was once supposed the red cells flowed through a second tube within the capillary.

Capillaries of Frog's Web.

The width of the still layer is

about that of the entire vessel, and is increased by cold,

decreased by heat. The rate of the flow in the centre is said to be 13 inches per minute by Volkmann, and as the rate through arteries is probably 700 inches per minute, he calculates that the entire area of the capillaries must be 400 times that of the arteries they spring from. Capillaries may be seen to contract either by elasticity after distension, or by muscularity, an endowment which the nuclei in the walls would suggest they possess. That the capillary circulation depends on the heart is argued on the following grounds: 1. The pressure on the sides of the veins, as in the arteries, is derived from the heart and must be sent through the capillaries. 2. Pressure on the main artery soon enfeebles the capillary circulation. 3. In fishes the heart forces the blood into the aorta through the capillaries of gills. And lastly, a pulse wave may be seen in the capillaries in some cases of debility, probably owing to a thin state of blood.

Capillary Force, or the "vis a fronte" produced by the interchanges between the blood in the capillaries and the tissues about them, is explained by Draper, who for many years has enunciated the following law: "If 2 fluids communicate with one another in a capillary tube, for the substance of which they have affinities of different intensities, movements will ensue; the liquid having the highest affinity will occupy the tube, and may even drive the other before it." This law has the advantage of universality, as it explains the movements of the sap in the vegetable kingdom, the force of which is shown by the following experiments: divide the stem of a vine, and tie a piece of bladder over the cut surface, when it will be burst by the sap ascending from the roots; and if the upper segment be placed in water, that fluid will be sucked up, as long as under the influence of light the formative action of the leaf goes on. The rate of the circulation in pulmonary capillaries is 5 times as great, so much chemical interchange occurring, as that in the systemic; and Draper therefore regards re

spiration as a most powerful cause of circulation. The exclusion of oxygen removes this cause of circulation, asphyxia resulting, as in drowning, inhalation of nitrogen, hydrogen, or other negatively injurious gases. This capillary force accounts for the following facts more clearly than can any vis a tergo, such as the heart: 1. The empty state of arteries after death. 2. Movement of blood in the early embryo and acardiac fœtus in animals without a heart, and after the heart has been removed or aorta tied. 3. The portal circulation. 4. Periodic and local variations, as those produced by mental emotions, blushing by shame, blanching by fear; local inflammations, congestions and fluxes, and the effect of cold; and lastly, such changes in the lungs as are produced in asphyxia and the first inspiration. This force also explains those occurrences expressed by the aphorism, "ubi stimulus ibi fluxus," of which the increased flow to a gland when about to secrete, and to a part when about to inflame after the application of an irritant, are examples. Many other instances will be adduced when we are discussing the pathology of inflammation, and it will be shown, that they are mainly due to the regulating influence of those cerebro-spinal and sympathetic nerves which constitute the vaso-motor system.

Erectile Tissue is but a mass of capillaries shorter and more freely communicating than usual. It is found in the corpora cavernosa of penis and clitoris, in the nymphæ, vagina, and nipple. Its turgescence is excited by either local irritation or by emotion conveyed through nervous system. Kölliker thinks its erection due to the removal of nervous influence, which had before kept the calibre of the arteries contracted by their muscular coat, which is always highly developed. This view is supported by the fact of priapism occurring in paraplegia, and by erection being retarded by cold, promoted by heat. Some abnormal examples of this tissue which I have met with, will be mentioned.

The Veins collect the blood from capillaries, and gradually enlarging, pour it through the cave into the right auricle. They are arranged in two sets-superficial, which have no corresponding artery; and deep, of which two usually accompany an artery, its venæ comites. When full they have a cylindrical form, but not so uniformly as arteries, and they are dilated into pouches above the valves. The coats are similar to those of arteries, but much thinner, and even transparent, the colour of blood appearing. In the cave near the heart, striped muscular fibres are continued from the auricle. Kölliker has not found muscular tissue in veins on the uterine side of placenta, in those of brain, dura mater, diploe, corpora cavernosa. At variable distances valves are found usually in pairs, and always with their free semilunar edge turned towards the heart. Valves are numerous in superficial veins, scarce in deep and large ones, and absent in the portal, pulmonary, and renal systems, and those of bones. Although there is three times as much venous as arterial blood, its proportionally slower circulation transmits about an equal quantity of each through a part in a given time. The rate of flow is very equable, as we see in venesection, and is more rapid the nearer it approaches the heart. That the heart affects the current is evident from its producing "the venous pulse," and which occurs mainly when the blood is thin, as in anemia, and by the flow being so much retarded by pressure on the main artery. This force is, however, but

of the accompanying artery.

of that

Respiratory movements influence venous flow, for during expiration the veins become turgid, as seen in dyspnoea, and are emptied by next inspiration, the blood rushing towards the partial vacuum. Sir D. Barry

showed this by inserting a tube in the jugular vein of a horse, when a coloured fluid was raised through it at each inspiration. This would only occur in the veins near the thorax, and would be prevented by the collapse