Section 18. Now the Essential Problem Which the Digestive Canal of

the rabbit solves is to get these insoluble, or quasi-insoluble, bodies into its blood and system. They have to pass somehow into the circulation through the walls of the alimentary canal. In order that a compound should diffuse through a membrane, it must be both soluble and diffusible, and therefore an essential preliminary to the absorption of nutritive matter is its conversion into a diffusible soluble form. This is effected by certain fluids, formed either by the walls of the alimentary canal or by certain organs called glands, which open by ducts into it; all these fluids contain small quantities of organic compounds of the class called ferments, and these are the active agents in the change. The soluble form of the carbohydrates is sugar; proteids can be changed into the, of course, chemically equivalent but soluble and diffusible the peptones; and fats and oils undergo a more complicated, but finally similar change.