The opening of the superficial vessels for the purpose of extracting blood constitutes one of the most common operations of the practitioner. The principal results, which we effect by it, are 1st. The diminution of the mass of the blood, by which the overloaded capillary or larger vessels of some affected part may be relieved; 2. The modification of the force and frequency of the heart's action; 3. A change in the composition of the blood, rendering it less stimulating; the proportion of serum becoming increased after bleeding, in consequence of its being reproduced with greater facility than the other elements of the blood; 4. The production of syncope, for the purpose of effecting a sudden general relaxation of the system; and, 5. The derivation, or drawing as it is alleged, of the force of the circulation from some of the internal organs, towards the open outlet of the superficial vessel. These indications may be fulfilled by opening either a vein or an artery.