Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/121

VASCULAR SYSTEM 105 forms : (1) as rhythmical contractions, such as have been seen in the vessels of a rabbit s ear or iu a bat s wing, which are inde pendent both of the pulse and of respiratory movements ; and (2) as persistent contractions, under the influence of the nervous system, which play an important part in the distribution of the blood. The amount of contraction of an artery will affect the pressure of the blood in its interior : it will accelerate or retard the rapidity of the blood current ; and it will regulate the supply of blood to the capillary area to which the vessel is distributed. By such arrange ments, also, the distribution of blood to various organs is regulated, thus establishing what has been termed a balance of local circula tions. For example, if the vessels in one organ remain permanently contracted, whilst those in a neighbouring organ are dilated, more blood will pass to the latter than to the former, and some end of physiological importance may be served. Thus physiological correlations may be established between the cerebral and thyroid circulations, the gastro- hepatic and the splenic circulations, and the distribution of blood in the lower extremities as related to the abdominal organs. -terial If a stethoscope be placed over a large arterj , a murmur, sound, uuds. or bruit will be heard, caused by the blood rushing through the vessel narrowed by the pressure of the instrument. The fluid escapes into a wider portion of the vessel beyond the point of pressure, and the sound is caused by the particles of fluid there being thrown into rapid vibration, not by vibrations of the wall of the vessel. Such sounds are favoured by a certain degree of elasticity in the walls of the vessel, by diminished peripheral resistance allowing the blood to flow away freely, and by a considerable difference of the pressure in the narrow and wide parts of the tube. They are always heard over an aneurism, when the arterial tube is dilated, and when pressure is applied to a large vessel. The placental souffle or bruit heard during pregnancy is a sound of this kind, arising from pressure on the widely dilated uterine arteries. In cases of insufficient aortic valves a double blowing murmur may be heard, the first being due to the rush of blood into the vessel caused by the ventricular contraction, and the second by the flowing back of the blood into the heart during diastole. Comp. p. 106 below. vpillary Capillary Circulation. The circulation in the capillaries may be rcula- readily studied by placing under the microscope any transparent on. membrane containing vessels, such as the web of a frog s foot, the mesentery, lung, or tongue of a frog, the tail of a fish or a tadpole, the wing of a bat, the third eyelid of the pigeon or fowl, the liver of a frog or a newt, the mucous membrane of the inner surface of the human lip, or the conjunctiva of the eyeballs and eyelids. Under favourable conditions the following phenomena may clearly be noticed. (1) The diameter of the finest capillaries is such as to per mit the passage of corpuscles in single file only, and it may vary from Wutf to rizvrriT-ij f an inch. (2) The average length is about -^ of an inch. (3) The number varies according to the degree of activity of the tissue, being numerous where nutritive processes are active, as in the liver and muscles. (4) They form networks or anastomoses, the form and arrangement of which are determined by the tissue elements. (5) In the smaller arterioles and venules, and in the capillaries, the current is continuous and there is no pulse. Owing to the elasticity of the larger vessels the intermittent movement of the blood caused by each ventricular contraction is in the capillaries transformed into a continuous flow. (6) In some of the larger vessels the current is more rapid than in others of equal calibre : that is to say, it is more rapid in small arteries than in small veins. (7) The current appears to have a uniform velocity in all ultimate capillaries of the same size. (8) Sometimes a slight acceleration of the rapidity, even in the smallest vessels, may be observed to follow each cardiac beat. (9) In a vessel larger than an ultimate capillary, so large as to permit the passage of several coloured corpuscles abreast, these may be seen travelling with great apparent velocity in the centre of the stream, whilst the colourless corpuscles move more slowly and with a rolling motion next the walls of the tube, in a layer of plasma called Poiseuille s space. The coloured corpuscles also remain sepa rate from each other, and do not exhibit any tendency to adhere together or stick to the walls of the vessels, whereas the colourless corpuscles do both, more especially after the membrane has been exposed for some time to the air, so as to excite the early stages of inflammation. Prof. D. J. Hamilton has shown that the nearer a suspended body approaches the specific gravity of the liquid in which it is immersed the more it tends to keep in the centre of the stream, and he states that the reason why the coloured corpuscles keep the centre and the colourless the sides of the stream is that the specific gravity of the former is the same or slightly greater than the blood plasm, whilst that of the colourless corpuscles is less. (10) If the calibre of an ultimate capillary be marked at the beginning of an observation, and again some time afterwards, it will frequently be noticed that it has become narrower or wider, indicating that contractility is one of the properties of capillaries. (11) The velocity is greater in the pulmonary than in the systemic capillaries. (12) The phenomenon known as diapedesis, or migration of the white blood corpuscles, first described by Waller in 1846, is readily seen in the mesentery of the frog after inflammation has been excited by exposure to the air for one or two hours (see fig. 20). It consists of the adhesion to the wall of the vessel of the colourless corpuscles and their protrusion through the wall into the surrounding tissues. Hering is of opinion that it is due partly to the filtration of the colloidal matter of the cell under blood-pressure. Diapedesis is of importance as constituting a part of the inflammatory process. The colourless cells become pus corpuscles (Cohenheim) ; see PATH OLOGY, vol. xviii. p. 365. (13) If a vascular membrane be ently irritated r whilst under the microscope, the capillaries become first slightly nar rowed and then di lated, crowded with corpuscles, whilst the blood - stream , becomes slower. By and by the stream oscillates and then alto gether stops. This constitutes stastis, FIG. 20. Small vessels of mesentery of frog, showing a part of the in- diapedesis of colourless corpuscles, w, w, vascular fli mnntorv urn walls ; aa Poi seuille s space ; r. r, red corpuscles ; I, I, y . V, colourless corpuscles adhering to wall ; c, c, colourless cess, and IS lol- corpuscles in various stages of extrusion ; /, /, extruded lowed by exudation corpuscles. (Landois and Stirling.) of the plasma of the blood, along with colourless corpuscles, and more rarely coloured corpuscles. The most important vital property of capillaries is, as already Vital mentioned, contractility, by which their calibre may be modified, proper- The protoplasm forming their walls contracts when stimulated, ties of Some investigators have supposed the nuclei to be active agents in capil- contraction ; but more probably the cell substance is the seat of laries. change. Oxygen causes the nuclei to swell, whilst carbonic acid has the opposite effect. Roy and Graham Brown attach much im portance to the active contractility of the capillaries as regulating the distribution of blood, now contracting, now relaxing, according to the needs of the tissues in their vicinity. Elasticity is also a characteristic of the capillary walls. The arrangement of the capillaries in an organ or tissue is adapted Arrange- to its functional activity. Where there is great functional activity ment of there is a rich plexus of capillaries, and in the converse case the capil- converse is also true. Contrast, for example, the capillary supply laries. in cartilage with that of muscle, or that of the grey matter of the nerve centres with that of the white matter (see PHYSIOLOGY, vol. xix. p. 23 sq.). But, in addition, the distribution of capillaries always corresponds to the intimate structural arrangements of the tissue or organ. So precisely is this the case that a good histologist is able to identify the organ from an injected preparation showing the vessels, although none of the ultimate histological elements of the organ or tissue are to be seen. In muscle, for example, the capillaries exist in the form of elongated meshes ; in connective tissue, such as is found beneath the skin, in an irregular network ; in the papillte of the skin, in loops ; and to form the glomeruli of the kidney in close reticulations (see NUTRITIOX, vol. xvii. p. 673, fig. 5). The movement in the capillaries is due to the force of the heart, as modified by the vessels (v is a tergo). Some have supposed that it is supplemented by an attractive influence exerted by the tissues (vis afronte) ; and the statement is supported by the observation that, when there is an increased demand for blood owing to active nutritional changes, there is an increase in the amount of blood flowing to the part, such as occurs, for example, in the mammary gland during lactation, and in the growth of the stag s horn. Such an attractive influence on the part of the tissues is quite conceivable as a force assisting in the inward flow of blood, acting along with capillarity ; but its amount is infinitcsimally small in comparison with the force exerted by the heart. The force of the heart is sufficient to drive the blood through the capillaries into the veins. When capillaries are examined in a transparent membrane of a living animal no pulse-like movement is visible. Owing to the elasticity of the vessels the pulse-wave has been almost, if not quite, extinguished, and what might have remained of it is destroyed by the great resistance offered by the numerous capillaries. If these and the arterioles be widely dilated, a pulse may appear in the veins, as occurs when the vaso-dilator fibres of the chorda tym- pani nerve are stimulated, causing a pulse-like movement in the veins of the sub-maxillary gland (see PHYSIOLOGY, vol. xix. p. 30). By increasing the extra-vascular pressure pulsations may occur in the capillaries (Roy and Graham Brown). The well-known throb bing in the finger when constricted by an india-rubber band and the throbbing of inflammatory swellings are examples of pulsation in capillaries. Venous Circulation. The walls of the veins are thinner, less Venous elastic, and more distensible than the walls of the arteries. They circtila- contain both elastic and contractile tissue, though to a smaller ex- tion. XXIV. 14 Attract ive in fluence oftissues. Capillary

pulse.