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TEMPORARY AND PERMANENT TEETH.
Fig. 1. The gums and outer plate of the bone are removed, showing the teeth of the infant, as they exist at the time of its birth; they are without roots, and contained in a capsule within the jaws.
Fig. 2. In this figure, also, the outer alveolar plate of the jaws has been removed to show the succession of teeth. This is the state at six years of age. The temporary teeth are all shed between the ages of seven and fourteen, and are supplied by the permanent teeth already nearly perfectly formed, and situated at the roots of the former.
Plate XXIX.- FORAMEN OVALE, AND DUCTUS ARTERIOSUS.
Fre. I. A view of the fætal heart; a, the ascending, b, the descending vena cava; C, the right auricle; d, e, f, mark the elevated ring of the foramen ovale, or the opening between the two auricles.
Fig. 2. The fætal heart; a, the pulmonary artery; b, b, its branches;c, the ductus arteriosus, or canal for transmitting the blood into, d, the aorta. As the lungs are useless in the fætus, unless as a “prospective contrivance," the heart has to carry on a single circulation only: the free communication between the two auricles identifies them as one cavity; and tne ventricles also force the blood into one vessel, the aorta.