e parts are related better than any verbal description can.
Between the coiling alimentary tube and the body walls is a space,
into which the student cuts when he begins dissecting; this is the
peritoneal cavity (pt.). A thin, transparent membrane, the mesentery,
holds the intestines in place, and binds them to the dorsal wall of
this peritoneal space.

Section 17. The food stuffs of an animal, the unstable compounds
destined ultimately to be worked into its life, and to leave it again in
the form of katastases (Section 13), fall into two main divisions. The
first of these includes the non-nitrogenous food stuffs, containing
either carbon together with hydrogen and oxygen in the proportion of
H2O (the carbo-hydrates), or carbon and hydrogen without oxygen
(the hydrocarbons). The second division consists of the nitrogenous
materials, containing also carbon, hydrogen, a certain amount
of oxygen, sulphur, and possibly other elements. Among the
carbohydrates, the commonest are starch and cellulose, which are
insoluble bodies, and sugar, which is soluble. The hydrocarbons,
fats, oils, and so on, form a comparatively small proportion of the
rabbit's diet; the proverb of "oil and water" will remind the student
that these are insoluble. The nitrogenous bodies have their type in
the albumen of an egg; and muscle substance and the less modified
living "protoplasm" of plants, a considerable proportion of the
substance of seeds, bulbs, and so on, are albuminous bodies, or
proteids. These also are insoluble bodies, or when soluble, will not
diffuse easily through animal membranes.

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 c