discontinued beyond a brief interval without fatal consequences, the latter such as may be suspended or even destroyed without involving general dissolution. Thus, although eight is important to comfort, it may be lost without affecting vitality; the hepatic function may be vicariously performed; even the renal secretion may be suspended for a considerable period without death: but the complete cessation of any of the essential functions of circulation, innervation, or respiration must be speedily followed by such a result. By the circulatory forces, a constant flow of blood is directed to and from all the parts; by the nervous system, an alternating effect is produced on the tissue-elements, whereby at one time they assimilate, at another disintegrate; by the respiratory apparatus, certain of the resultant products are incessantly eliminated. These three complemental functions are so interdependent that the complete interruption of either necessarily leads to arrestment of all, and consequent death.
Human blood is of a highly complex nature, as through it the textures receive all the materials adequate to their continued maintenance and repair. Its chemical composition is never definite, varying in different individuals and in the same individual on different occasions. The relative uniformity, however, of some of its physical characters is indispensable to its vital efficiency. It is semi-solid, containing innumerable white and red corpuscles, the latter constituting nearly one half its mass. The absolute number of these corresponds with the degree of general vitality; their local aggregation fluctuates with varying contingencies.
This fluid is the seat of two distinct modes of motion—a sensible circulation through the heart and vessels, and a subtiler interchange with tissue-elements. Several causes conspire toward its circulatory mass-motion, the heart's action being a sine qua non. The molecular motions being invisible, an explanation of their modus operandi must be partly hypothetical. There are, however, certain associated phenomena admitting of direct observation under certain circumstances which serve to throw light on the physico-vital relations of the blood. Thus, besides its general distribution, it is subject to local variations in the total quantity of its mass, and in the relative proportion of its various constituents. As there are means of artificially exciting preternatural activity of the circulation to a recognizable extent, in parts open to observation, during the minimum degree of vitality, such a possibility affords a reliable method of infallibly deciding in any particular case as to the existence or non-existence of this vital process.
Tissues are divisible into vascular and non-vascular, according to the mode and extent of their nutritive supply. The latter, being destitute of capillaries, receive their nourishment from the neighboring vessels by endosmosis. The former are pervaded by those minute vessels, which admit red corpuscles in a lesser or greater number, ac-
