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 resistance occasioned by flexures in pipes and rivers, the propagation of an impulse through an elastic tube, and some of the phenomena of pulsations. This paper was preparatory to the second, “On the Functions of the Heart and Arteries,”—the Croonian lecture for 1808—in which he showed more clearly than had hitherto been done (1) that the blood pressure gradually diminishes from the heart to the periphery; (2) that the velocity of the blood becomes less as it passes from the greater to the, smaller vessels; (3) that the resistance is chiefly in the smaller vessels, and that the elasticity of the coats of the great arteries Comes into play in overcoming this resistance in the interval between systoles; and (4) that the contractile coats do not act as propulsive agents, but assist in regulating the distribution of blood.

The next epoch of physiological investigation is characterized by the introduction 'of instruments for accurate measurement, and the graphic method of registering phenomena, now so largely used in science. In 1825 appeared E. and Wilhelm Weber’s (1804–1891) Wellenlehre, and in 1838 Ernest Weber’s (1795–1878) ''Ad Notat. Anatom. et Physiolog.'' i., both of which contain an exposition of E. H. Weber’s schema of the circulation, a scheme which presents a true and consistent theory. In 1826 Jean Louis Marie Poiseuille invented the haemadynamometer. This was adapted with a marker to a recording cylinder by, Ludwig in 1847, so as to form the instrument named by Alfred Volkmann (1801–1877) the kymograph. Volkmann devised the haemadromometer for measuring the velocity of the blood in 1850; for the same purpose Vierordt constructed the haematachometer in 1858; Chauveau and Pierre Lortet (1792–1868) first used their haemadromograph in 1860; and lastly, Ludwig and Dogiel obtained the best results as regards velocity by the “stream-clock” in 1867. As regards the pulse, the first sphygmograph was constructed by Karl Vierordt (1818–1884) in 1856; and Etienne Marey’s form, of which there are now many modifications, appeared in 1860. In 1861 Jean Chauveau (b. 1827) and Marey obtained tracings of the variations of pressure in the heart cavities (see below), by an experiment which is of great historical importance. During the past twenty-five years vast accumulations of facts have been made through the instruments of precision above alluded to, so that the conditions of the circulation, as a problem in hydrodynamics, have been thoroughly investigated. Since 1845, when the brothers Weber discovered the inhibitory action of the vagus, and 1858, when Claude Bernard (1813–1878) formulated his researches showing the existence of a vaso-motor system of nerves, much knowledge has been acquired as to the relations of the nervous to the circulatory system. The Webers, John Reid (1816–1895), Claude Bernard and Carl Ludwig (1809–1849) may be regarded as masters in physiology equal in standing to those whose researches have been more especially alluded to in this historical sketch. The Webers took the first step towards recognizing the great principle of inhibitory action; John Reid showed how to investigate the functions of nerves by his classical research on the eighth pair of cranial nerves; Claude Bernard developed the fundamental conception of vaso-motor nerves; and Ludwig showed how this conception, whilst it certainly made the hydraulic problems of the circulation infinitely more complicated than they were even to the scientific imagination of Thomas Young, accounted for some of the phenomena and indicated at all events the solidarity of the arrangements in the living being. Further, Ludwig and his pupils used the evidence supplied by some of the phenomena of the circulation to explain even more obscure phenomena of the nervous system, and they taught pharmacologists how to study in a scientific manner the physiological action of drugs.

The unicellular animal immersed in water absorbs nutritive matter and oxygen, and excretes waste materials with its whole surface. Owing to the small mass of the protozoa The the metabolic products can penetrate throughout the whole. With the evolution of the multicellular organs of the metazoa and the division of physiological labour

a circulatory mechanism became of immediate need. A double-layered animal like the common water polype Hydra can. exist, it is true, without such a mechanism, but communities of polypes, such as the sponges, form channels for the circulation of water. With the development of the three-layered animal the coelom or body cavity arose by the splitting of the mesoderm, and it was in this body cavity that the evolution of the circulatory system took place, an evolution which finally became perfected in the higher members of the metazoa into a closed vascular system filled with red blood. The evolution of the red matter, haemoglobin, as a special carrier of oxygen was necessitated by the increasing mass and muscular activity of the higher animal, in comparison with the size of the oxygen absorbing surface—the gill or lung. The blood vascular system of the in vertebrata such as the Arthropoda and Insecta, is not generally a closed system, but consists of a pulsatile heart whence proceed arteries which open into lacunar spaces forming part of the coelom. The lacunae exist between the organs and tissues of the body, and the blood from these spaces is returned to a venous sinus whence the heart draws its supply through valved openings. The movements of the animal help to return the blood from the tissue spaces to the heart, while the heart by its rhythmic contraction drives the blood into the arteries. Somewhere in the course of this system are placed the gills and renal organs, and it appears to be a matter of indifference, whether the gills be placed on the arterial or venous side of the system, both arrangements being found in different types. In some types (mussel, earthworm), the whole blood passes through the renal organs at each circulation, in others (crayfish) only parts. In the earthworm the vascular system is closed, the arteries and veins being connected by capillaries in place of lacunae. The movement of tissue juices may be maintained by physico-chemical forces alone, e.g. by the forces of osmosis and adsorption, as is seen in the movements of sap in the vascular bundles of plants, in the streaming of protoplasm in the plant cell and in the marvellous rhythmic to-and-fro movements of the richly granular juice contained in the veins of the spreading protoplasmic sheet of myxomycetes. Such agencies come into play in the lacunar or capillary part of the circulation of the metazoa and are assisted by the movements of the body wall and of the alimentary organs. The evolution of a special pumping organ, the heart, associated with the aeration of the body fluids in the gills, led to the perfection of the efficient system of circulation which is found, in the vertebrata.

The blood is to be regarded as alive in as strict a sense as any other- component of the living body. It is a tissue consisting of mobile elements—the blood corpuscles—and a plasma—a colloidal albuminous fluid which is analogous to the more solid intercellular material of other tissues. The primary sources of its elements are the blood-forming organs—the bone marrow, the haemolymph and lymphatic glands and other lymphatic tissue, and the spleen. It circulates as the middleman between the tissues, conveying from the alimentary canal the products of digestion—sugar, fat, amino acids and salts; oxygen from the lungs; carbonic acid, urea and other waste products of the tissues to the lungs and kidneys; internal secretions from one organ to another; and acts not only as a carrier, but deals with the material remitted to it on the way. Une other function of the blood, a most important one, must not be omitted, that of defence against the invasion of bacteria and their toxins, and other parasites.

The blood is contained in a continuous system of vessels; arteries lead from the heart and divide into a multitude of