Page:The American Cyclopædia (1879) Volume IV.djvu/621

 CIRCULATION 609 vessels of the limb, so as to return in a full stream by the femoral vein, it is found that the force requisite to accomplish this result, as measured by a pressure gauge connected with the syringe, is from 120 to 130 millimetres of mercury. Prof. Sharpey of London found that an injection might be made to pass in this way from the mesenteric artery through both the two successive capillary systems of the intestine and the liver, and return in a full stream by the vena cava, under the pressure of a column of mercury 130 millimetres in height. But we have already shown that the blood is habitually subjected in the arteries of the living animal to an average pressure of 150 millimetres. This force accordingly is amply sufficient to account for the movement of the blood through the capillaries generally, from the arteries to the veins. The rapidity of movement of the blood in the capillaries is much less than in the arteries. This is due to the fact that the extent of vascular surface with which the blood comes in contact on en- tering the capillary system is vastly increased owing to the small size and great number of. the vascular canals through which it now flows; and consequently the mechanical re- sistance to its passage is increased in corre- sponding ratio. It is estimated, from data de- rived from microscopic inspection of the capil- lary circulation in transparent tissues, that the rate of movement of the blood in these vessels is about ^ of an inch per second. It must be remembered, however, that while the capillary vessels are excessively numerous, and their united calibre therefore very large, their length, on the other hand, is very small. The effect of this anatomical arrangement accordingly is to disseminate a comparatively small quantity of blood, while it is contained in the capillary vessels, over a very large space ; so that the physiological and chemical reactions taking place between the blood and the substance of the tissues are accomplished almost instanta- neously. Furthermore, although the rate of movement of the blood in these vessels is very slow, yet as the distance to be passed over be- tween the arteries and the veins is very small, the blood requires but a few seconds to trav- erse the capillary system, and to commence its returning passage by the veins. Movement of the Hood in the veins. In the veins the blood is gradually collected from the periphe- ral parts of the circulatory system and returned to the right side of the heart. Beginning by small rootlets directly connected with the ca- pillaries, the veins constantly unite with each other into larger branches and trunks, converg- ing in this way toward the centre of the circu- ation, until they terminate in the right auricle of the heart by two great canals, one coming from the upper and one from the lower parts of the body, and named accordingly the ve- na, cava superior and the vena cava inferior. ~ ie walls of the veins are thinner than those ' the arteries, and are less distensible and less elastic ; but they have great strength owing to the large proportion of white fibrous tissue in their structure, and are able to resist without laceration an equal or even greater pressure than the arteries of corresponding size. They are provided at various intervals with mem- branous semi-lunar valves, which open toward the heart and shut backward in the opposite direction. The main cause of the movement of the blood in the veins is the pressure from behind exerted by the blood as it is carried through the capillary circulation. It thus fills the rootlets and smaller branches of the veins with a constantly increasing supply, and urges the blood which they already contained on- ward to the heart. If this were the only force at work, the venous system would after a time become so filled with blood that its resistance would counterbalance the pressure of blood from the arteries, and the circulation would come to an end by simple engorgement of the veins. But in point of fact the right ventricle of the heart at each pulsation discharges into the pulmonary artery, and, as it were, lifts away from the venous system a portion of the blood which had accumulated in it. Thus the veins are protected from engorgement, and their backward resistance is kept always in- ferior to the pressure by which the blood en- ters them from the capillaries. The venous circulation is also much facilitated by the al- ternate contraction and relaxation of the vol- untary muscles. When any one of these muscles contracts, it increases in thickness ex- actly in proportion as it diminishes in length. The effect of this lateral swelling of the muscles in activity is to compress the veins situated between them, and the blood is thus forced out of the compressed portion. The valves of the veins, already mentioned, prevent the blood from regurgitating toward the ex- tremities, and it is consequently driven onward toward the heart, in which direction alone the passage for it is clear. When the muscles relax, the corresponding portion of the vein is again rapidly filled with blood coming from the extremities, and the circulation goes on with increased facility from the circumference toward the centre. Variations of the circula- tion in different parts. We know that the capillary circulation in various parts of the body is subject to marked variations, depen- dent on external or internal causes. Thus a mustard poultice, or a sponge moistened with water of ammonia, applied to the skin, will cause a local and circumscribed redness, due to an increased capillary circulation in the part. The face will become flushed or pallid under the influence of mental emotion, and cold or hot applications will cause a similar change of color in those parts of the skin with which they are brought in contact. Even the inter- nal organs exhibit fluctuations of the circula- tion of the same kind, to a marked degree. When a glandular organ is about to enter into active secretion, it is the seat of a kind of