BLOOD-VESSELS. The blood from which the tissues of the body obtain the material of their nourishment is conveyed from one part of the body to another by means of branched tubes which are named Blood-Vessels. It is carried along these vessels by the impulse given by the action of the Heart. [HEART.] The vessels which carry the blood from the heart are called Arteries. [ARTERY.] Those which return the blood to the heart are named Veins. [VEIN.] Whilst a very generally diffused network of Blood-Vessels exist, connecting the arteries and veins, which are called Capillaries. [CAPILLARY VESSELS.] The Blood-Vessels, whatever may be their ultimate destination, seem to originate in the same manner. Observations on this subject have been made by Schwann and Kffiliker in Germany, and by Professor Paget in this country. The observations of the two former were made on the development of the vessels in the germinal membrane of the egg, and on the capillary blood-vessels of the tail of the Lire's of a frog. 3Ir. raget'a observations were made on the tissues of the foetal sheep. According to these observers it appears that these vessels originate from nucleated cells similar to those which at first constitute the different parts of the embryo. The cell-wall or external envelope of these cells shoots out into slender poiutod processes, such as is seen in the forms of stellate vegetable tissue. The projections from neigh bouring cells encounter each other, and becoming organically united, the intervening walls between the two projections are absorbed, and thus a continuous tube is produced. In cases where new vessels are produced in the neighbourhood of old ones, the stellate cells are formed in the new part, and projections are formed in the old capillary vessels 'which unite with the new ones, and thus the circulation is re-esta blished. The projections when first united are solid and very slender,
but eventually the intervening substance disappears and the vessels attain a uniform calibre. .In growing parts where the web of vessels is kept up, new ones are constantly being added by the development of stellate cells in the interstices of the previous web. Whilst the capillaries early attain the development at which they remain, those vessels which are to become arteries or veins on either side of the capillary vessels go on increasing in size till they acquire the special membranes or coats which distinguish these parts of the circulating system. This explanation seems however only applicable to the smaller veins and arteries, as the observations of Kolliker would seem to show that the larger Blood-Vessels may take their origin in the same manner as the heart, in which organ there is first an agglomeration of cells, the interior ones of which become soft, and at last disappear, whilst the outside ones become firmer and constitute the outer walls. On this subject further observations are wanting.
(Sharpey, Quail; Elements of Anatomy; Schwerin, Microscopical Researches into the Accordance in the Structure and Growth of Animals and Plants, translated by H. Smith; ECilliker, Ilandlouch der Gcwebe lehre der .Menschen ; Paget, Supplement to .31itller's .Physioloyy, by Raley and Kirkes.)