The reader can more easily follow my descriptions of vessel shape if he will bear in mind the following. I describe a segment of vessel profile according to (a) curving quality and (b) angle or flaring quality. Under (a) a profile may be outcurved, rounded, straight or incurved; under (b) it may be flaring, vertical, or insloping. A vessel wall or any part of it may, for example, be at once slightly rounded and vertical, or slightly rounded and flaring, or straight and insloping.