Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/119

VASCULAR SYSTEM 103 elastic tube, formed, say, of india-rubber, the fluid does not issue from the other end in a series of jets, which would be the case if the tube were rigid, but it flows continuously, because during the pause between the successive strokes the outflow still continues. Consequently a continuous flow is kept up in elastic tubes when the time between two strokes is shorter than the duration of the outflow after the first stroke. If the finger be placed on any part of such a tube, and more especially near the pump, an expansion and relaxation will be felt with each stroke. Further, if the right fore-finger be placed over the tube near the pump, and the left fore-finger over a more distant portion, a stronger impulse will be felt with the right than with the left finger. Thus a wave is transmitted along the tube, diminishing in amplitude as the dis tance from the pump increases. We must distinguish between the transmission of this wave (an oscillatory movement or change of form in the column of fluid) and the transmission of the current, that is, the translation of a mass of fluid along the tube. In elastic tubes the current is much slower than the transmission of the wave. The progress of the wave of oscillation may be traced graphically by an apparatus devised by Marey. 1 E. H. Weber gives the velocity of waves in elastic tubes at 36 76 feet per second, and Bonders states it at 36 to 42J feet. Increase of pressure in the tube appears to lessen the velocity of the wave. The specific gravity of the liquid also affects the velocity : thus the wave is propa gated four times more slowly in mercury than in water. 2iieral The blood-vessels consist, as already stated, ructure of the arteries, the capillaries, and the blood- veins. (a) The ultimate or most minute FlG - H. Capillaries of ssels. capillaries have the simplest type of structure i^T^mtfclf ina fntfied (fig. 14), consisting of tubes formed of a single layer of transparent, thin, nucleated, endo- thelial cells, joined at their margins. A perfectly fresh capillary does not show the edges of the cells, owing to the uniform re fractive property of the wall of the tube. The nuclei show an internuclear plexus of fibrils. The cement substance uniting the cells is stained black by a ^ per cent, solu- and :uted on by nitrate of silver, to show that it is composed of flat tened cells ; &, a smaller vessel showing the same ; c, a small artery or vein showing trans- capillary from pia mater of sheep s brain. tion of nitrate of silver. Here and there minute dots or slits may be seen, which have been supposed by some to be openings (stomata or stigmata). In the transparent parts of animals, such as the web of the frog s foot, the mesentery and the lung of the frog, and the tail of a fish, the blood may be seen (fig. 15) flowing through the capil lary network from the arteries into the veins. The current is rapid in the small arteries, less rapid in the veins, and slow in the capillaries. It is also fastest in the centre of the vessel and slowest near the wall. The colourless corpuscles of the blood may be seen to pass from the centre of the stream to the FIQ 15 __ c 1)lo0(l . vessels in margins, to adhere to the inner web O f a frog - s foot) a s seen with surface of the blood-vessel, and oc- the microscope. The arrows in- casionally to pass through the coats dicate the course of the blood. of the more minute vessels, appear- ( ing in the surrounding tissues as migratory cells. Capillaries form networks (fig. 15), which vary much in the size and closeness of the meshes, according to the degree of activity of the tissue elements. (b) An artery has three coats, an inner or elastic, a middle or muscular, and an external or areolar (fig. 16). The inner coat is formed of two layers, one of pavement epithelium, sometimes called endothelium, and the other composed of fine elastic fibres interlacing, or of a fine membrane per forated with holes of various sizes (fenes- trated membrane of Henle). In some vessels there is a thin layer of connective tissue between the epithelium and the elastic layer. The middle coat is formed of a layer of non-striated muscular fibres circularly disposed around the vessel, F IG . ic.. An artery of inter mixed with numerous elastic fibres con- mediate size, a, a, open- nected with the perforated membrane of "8* of branches and posi- the inner coat. The outer coat consists of tl(m - " connective tissue, mixed also with elastic fibres. In the aorta there is a considerable amount of sub-epithelial connective tissue (Schafer), and the elastic 6, ft, ?), muscular coat show ing transverse nuclei ; c, c, coat of areolar tissue. 1 Figured in M Kendrick s Outlines of I hysioloyy, pp. 338-339. coat attains great thickness and strength. As a general rule, the smaller arteries show a considerable development of the muscular coat, whilst in the larger it is the elastic coat that attains the pre ponderance. In many of the larger arteries there are longitudinal muscular fibres at the boundary of the middle and inner coats. Whilst the circular fibres, on contracting, must narrow the calibre of the artery, the longitudinal may tend to keep the vessel open. In the external coat of the larger arteries minute vessels, vasci vasorum, exist for the nourishment of the tissue elements of the arterial wall. (c) The veins have similar coats to those of an artery, with differ ences in detail. The elastic layer is less developed in the internal coat ; the middle coat is much thinner and has less elastic tissue, but more connective tissue. Many veins have semilunar folds of the internal coat strengthened with fibrous tissue, forming valves. In some veins (iliac, femoral, umbilical) longitudinal muscular fibres are found in the inner part of the middle coat ; in the inferior vena cava, hepatic veins, and portal veins these longitudinal fibres are external to the circular coat ; in the superior vena cava and upper part of the inferior vena cava the circular coat is wanting ; and the veins of the pia mater, brain and spinal cord, retina, bones, and the venous sinuses of the dura mater and placenta have no muscular tissue (Schafer). Valves exist in the larger veins only, especially in those of the limbs ; they are not found in the veins of the viscera, of the cranium and vertebral canal, or of the bones, nor in the um bilical vein. (d) The arterioles and venules are the small vessels, simpler in structure than the larger above described, but containing the same elements. In the smallest veins the elastic layer has disappeared, and the muscular layer is also very thin. Sometimes the muscular layer is represented only by a single layer of contractile cells, and in such minute vessels the outer coat and elastic layer have also disappeared, so that the vessel is merely a tube composed of pave ment cells with a few elongated fusiform muscular cells twisted around it. Even in the smallest vessels the following differences may be observed between arterioles and venules: " The veins are larger than the corresponding arteries ; they branch at less acute angles ; their muscular cells are fewer, and their epithelium cells less elongated ; the elastic layer of the inner coat is always less marked, and sooner disappears " (Schafer). (c) Tlie cavernous spaces, as existing in erectile tissues (cm-pus cavcrnosum of the penis), consist of the anastomosis of large veins of unequal calibre. The walls and partitions have numerous per forations ; threads of delicate tissue, covered with epithelium, pass through the cavities ; and the walls are strengthened by connective tissue. Similar structures connected with arteries form the carotid gland of the frog and the coccygeal gland of man. The physical properties of blood-vessels are cohesion and elas ticity. The cohesion is great and the elasticity is small and perfect. The walls of blood-vessels have the property of contractility, by which alterations take place in the calibre of the vessel, and con sequently in the amount of blood supplied to a part. Arterial Circulation. The arterial walls are both muscular and elastic, the muscular coat predominating in the smaller, whilst the elastic coat is strong in the greater arteries. The chief function of the elasticity of the greater vessels is to transmute the unequal movement of the blood in the large arteries, caused by the inter mittent action of the ventricle, into a uniform flow in the capil laries. Thus, when the ventricle contracts, it propels a certain amount of blood into the elastic aorta, which expands in all direc tions. On the commencement of the diastole of the ventricle the vis a tergo is removed ; the aorta owing to its elasticity recoils, so as to close, on the one hand, the semilunar valves and, on the other, to force part of its contents into the vessels farther onwards. These, in turn, as they already contain a quantity of blood, expand, recover by an elastic recoil, and transmit the movements with diminished intensity. Thus the blood is driven along the vessels by the action (1) of the ventricular systole, and (2) of the elastic recoil of the Avails of the vessels occurring during the intervals between the ventricular systole (see p. 104 below). 13y these actions a series of movements, consisting of expansions and contractions, gradually diminishing in amplitude, pass along the arterial system from the greater to the smaller vessels, the latter becoming, as already pointed out, less and less elastic. These expansions and relaxations of the arterial wall, passing along like a wave, con stitute the pulse. The pulse therefore represents merely the trans mission- of an undulating movement of the blood, not its pro gression in the vessels. The undulations of the pulse travel at the rate of 354. ^ inches per second, about 30 times faster than the movement of the blood, which in the carotid artery of the horse has been estimated to travel 11*8 inches per second. The pulse can be registered graphically by means of a SPHYOMO- OKAPH (q.v.). Yierordt constructed the first sphygmograph in 1855, substituting for a column of fluid a lever placed on the pulse, which communicated with a system of levers and thus amplified the movement. Of those subsequently devised the best form is that of Marey, invented in 1861. It consists essentially of a long Proper ties of blood vessels. Arterial circula tion. Pulse. Sphyg-

mograph.