variable depth in the region of 1–1.5 cm. In part, it is a result of the arched shape of the lateral lower profile of the walls and sole, so that when an unloaded hoof touches a firm ground surface, there is only contact at the toe and heels (active contact). A loaded hoof has a much greater area of ground contact (passive contact), covering the lower wall edge, most of the sole, bars, and frog. Active contact areas can be seen as slightly protruding spots in the walls and in the callused sole. The shape changes in a loaded hoof are complex. The plantar arch flattens, the solar concavity decreases in depth, and the heels spread. The hoof diameter increases to a 'dilated' configuration and P3 drops marginally into the hoof capsule. There is some recent evidence that a depression takes place in this phase, with blood pooling ('diastolic phase') mainly into the wall corium. When unloaded, the hoof restores its 'contracted' configuration, the pressure rises and the blood is squeezed out ('systolic phase'). There is a secondary pumping action with the flexion of the foot as it is raised. The hoof mechanism ensures effective blood circulation into the hoof, and it aids general circulation. Time-related changes of the hoof Hooves are a plastic structure and their time-related, very complex changes can be considered in the short term (days/weeks) and over the horse's lifespan. Hoof changes in the short term Just like the cornified layer of epidermis and of any mammalian nail, the hoof capsule is created only from the epidermis, the outer living layer of the skin. From a microscopic point of view, the epidermis is a multi-layered, specialised cornifying epithelium. It overlays the dermis, and it is separated from it by a basal lamina. It has no blood vessels, and living cells acquire their oxygen and nutrients by fluid exchanges and molecular diffusion from the underlying dermis, flowing into microscopical spaces among individual cells. Products of metabolism are cleared by the reverse of this process. Epidermis growth takes place by mitotic activity in its deepest layer, into the basal layer, with slow outward migration and maturation of cells. As these cells approach the surface, special proteins accumulate into their cytoplasm, then the cells die and 'dry', into microscopic, tightly-connected individual layers, composed mainly of keratin. The resulting 'dead' superficial layer serves a protective function, saving underlying living tissues