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    she appears to us to do so , it is only because we do not understand her . ' The earliest instance in which the movement of the fluids has teen seen is in the tribe of infusoria called Vorticellae ; creatures which fix themselves to other bodies by a kind of stalk , and have more the appearance of flowers than animals . Under the improved microscope

    two opposite currents have been seen flowing in the stalk ; one upwards , the other downwards . Somewhat higher in the scale of infusoria there is a distinct appearance of vessels . In insects the mode by which the circulation was effected was very imperfectly understood till very recently ; Mr . Bowerbank has now discovered that all down the dorsal vessel of the insect , and at regular intervals , are doable valves . The dorsal vessel is seen to contract , and the blood which can

    be observed through the transparent vessels flows in jets answering to its contractions . This vessel is probably the engine that works the current , and appears to be an extended chain of hearts . One step beyond this and every thing connected with the circulation becomes clear . Two distinct sets of vessels are distinguished , with a third organ interposed between them . This organ is the heart , the two sets of vessels are , the veins carrying blood to the heart , the arteries

    carrying blood from the heart . The heart at this early stage is extremely simple ; it consists of two bags which communicate with each other ; one receiving the blood from the veins , called the auricle , the other propelling it into the arteries , called the ventricle . The artery when it springs from the ventricle is a large trunk , it divides and subdivides , as it carries out the blood to the system till it completely supplies every part ; its ultimate branches are called the capillaries ,

    and they are so minute and so numerous , that the point of the finest needle cannot pierce the skin without wounding some of them . Where the capillary arteries end the capillary veins begin . These two sets of vessels communicale freely with each other . The capillary branches of the vein gradually becoming larger and larger at length terminate in a large trunk which returns the blood to the auricle , the auricle transmits it to the ventricle , the ventricle propels it into the

    artery , and so the circulation goes on . But it is requisite thai air should get at the blood to renovate it ; for in affording nourishment and stimulus to the different organs , it at length loses all the nutritive and stimulant properties it possesses . In the four highest tribes of animals and in man the blood is aerated by means of lungs in the case of those animals which breathe in air , and gills in the case of those which respire in water . Respiration so performed requires the structure of the heart to be complicated by the addition of another

    artery . This second artery conveys the blood , not to the system , but to the lungs , called the pulmonary artery ; while the artery which conveys it to the system is called the aorta . It also requires a set of veins to carry the renovated blood back to the heart . The more perfectly the blood is aerated , the stronger and more vigorous are all the actions of the economy . In reptiles and fishes it is only partially aerated ; and their hearts are single , consist , that is , of one auricl ? and one ventricle . In birds and the mammalia it is double ; but it is best ( passing over the lower animals ) to give a description of the circulation as it exists in man . We shall give it in Dr . Smith ' s own

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    Dr . Southwood Smith on the Animal Economy . 125

• Citation
  • Monthly Repository (1806-1838) and Unitarian Chronicle (1832-1833), Feb. 2, 1833, page 125, in the Nineteenth-Century Serials Edition (2008; 2018) ncse.ac.uk/periodicals/mruc/issues/vm2-ncseproduct2608/page/57/