All these facts point to the conclusion, that it is the tissues, and not the blood, which become primarily loaded with carbonic acid, the latter simply receiving the gas from the former by diffusion; and that the oxygen which passes from the blood into the tissues is at once taken up in the same combinations, so that it is no longer removable by diminished tension. The production of carbonic acid in the muscle is not directly dependent on the consumption of oxygen. The muscles produce carbonic acid in an atmosphere of hydrogen. What is true of muscle is true also of other tissues and of the body at large. Oxygen helps to wind up the vital clock; but once wound up, the clock will go on for a period without further winding (Pflüger). To sum up, then, the result of respiration in its chemical aspect. As the blood passes through the lungs, the low oxygen tension of the venous blood permits the entrance of oxygen from the air of the pulmonary alveolus, through the thin alveolar wall, through the thin capillary sheath, through the thin layer of blood plasma, to the red corpuscles, and the reduced hæmoglobin of the venous blood becomes wholly, or all but wholly, oxy-hæmoglobin. Hurried to the tissues, the oxygen, at a comparatively high tension in the arterial blood, passes largely into the tissues, in which the oxygen tension is always kept at an exceedingly low pitch, by the fact that the tissues, in some way at present unknown to us, pack away, at every moment, into some stable combination each molecule of oxygen which they receive from the blood. With much, but not all, of its oxy-hæmoglobin reduced, the blood passes on as venous blood. How much hæmoglobin is reduced will depend on the activity of the tissue itself. The quantity of hæmoglobin in the blood is the measure of limit of the oxidizing power of the body at large; but within that limit the amount of oxidation is determined by the tissue, and by the tissue alone. The skin is an excretory tissue, and consists principally of two layers, an external covering of epithelium, termed the cuticle or epidermis, and a layer of vascular tissue, named the corium derma or cutis vera. The integument serves (1) for the protection of deeper tissues, (2) as a sensitive organ in the exercise of touch, (3) as an excretory organ, (4) as an absorbing organ, (5) for regulating the temperature of the body. Within and beneath the corium are imbedded several organs with special functions, namely, sudoriferous or sweat glands, sebaceous or fat glands, and hair follicles; and on its surface are sensitive papillæ. The so-called appendages of the skin, the hair and nails, are modifications of the epidermis.