984. But in the capillary vessels there exists only blood. Universally and invariably before the blood passes from under the influence of the capillary vessels it has ceased to be blood. Arterial blood is conveyed by the carotid artery to the brain; but the cerebral arteries do not deposit blood, but brain. Arterial blood is conveyed by the capillary arteries to bone; but the osseous capillaries do not deposit blood, but bone. Arterial blood is conveyed by the muscular arteries to muscle, but the muscular capillaries do not deposit blood but muscle. The blood conveyed by the capillaries of brain, bone, and muscle is the same; all comes alike from the systemic heart, and is alike conveyed to all tissues; yet in the one it becomes brain, in the other bone, and in the third muscle. Out of one and the same fluid are manufactured cuticle, and membrane, and muscle, and brain, and bone; the tears, the wax, the fat, the saliva, the gastric juice, the milk, the bile, all the fluids, and all the solids of the body (310). 985. These phenomena are wholly inexplicable on any known mechanical principles. It is equally impossible to refer them to mere chemical agency, or to any properties of dead matter. We are therefore under the necessity of referring them to a principle which, for the sake of distinguishing it from anything mechanical or chemical, we term vital. As the actions which take place between the integrant particles of bodies, giving rise to chemical phenomena, are referred to one general principle, termed chemical affinity, so the actions which take place in living bodies, giving rise to vital phenomena, may be referred to one general principal, termed vital affinity. The term explains nothing, it is true, it merely expresses the general fact; but still it is convenient to have a term for the expression of the fact. The property itself will ever remain an ultimate fact in physiology, however exactly the limits of its agency, and the laws according to which it modifies the mechanical and chemical relations of the substances subjected to its influence, may hereafter be ascertained; just as chemical affinity will ever be an ultimate fact in physics, whatever discoveries may yet be made of the extent of its agency and of the conditions on which its action depends.