[18] The work of some kinds of moulds may be apparent to the eye, as in the growths that form on old leather and stale bread and cheese.  That of others goes on unseen, as when acids are formed in stewed fruits.  Concerning the work of the different kinds of moulds.  Troussart says:  “Mucor mucedo devours our preserves; Ascophora mucedo turns our bread mouldy; Molinia is nourished at the expense of our fruits; Mucor herbarium destroys the herbarium of the botanist; and Choetonium chartatum develops itself on paper, on the insides of books and on their bindings, when they come in contact with a damp wall.”—­Troussart’s Microbes, Ferments, and Moulds.

[19] “The physiological wear of the organism is constantly being repaired by the blood; but in order to keep the great nutritive fluid from becoming impoverished, the matters which it is constantly losing must be supplied from some source out of the body, and this necessitates the ingestion of articles which are known as food.”—­Flint’s Text-book of Human Physiology.

[20] Glands.  Glands are organs of various shapes and sizes, whose special work it is to separate materials from the blood for further use in the body, the products being known as secretion and excretion.  The means by which secretion and excretion are effected are, however, identical.  The essential parts of a gland consist of a basement membrane, on one side of which are found actively growing cells, on the other is the blood current, flowing in exceedingly thin-walled vessels known as the capillaries.  The cells are able to select from the blood whatever material they require and which they elaborate into the particular secretion.  In Fig. 47 is illustrated, diagrammatically, the structure of a few typical secreting glands.  The continuous line represents the basement membrane.  The dotted line represents the position of the cells on one side of the basement membrane.  The irregular lines show the position of the blood-vessels.

[21] Tablets and other material for Fehling and additional tests for sugar can be purchased at a drug store.  The practical details of these and other tests which assume some knowledge of chemistry, should be learned from some manual on the subject.

[22] The Peritoneum.  The intestines do not lie in a loose mass in the abdominal cavity.  Lining the walls of this cavity, just as in a general way, a paper lines the walls of a room, is a delicate serous membrane, called the peritoneum.  It envelops, in a greater or less degree, all the viscera in the cavity and forms folds by which they are connected with each other, or are attached to the posterior wall.  Its arrangement is therefore very complicated.  When the peritoneum comes in contact with the large intestine, it passes over it just as the paper of a room would pass over a gas pipe which ran along the surface of the wall, and in passing over it binds it down to the wall of the cavity.  The small intestines are suspended from the back wall of the cavity by a double fold of the peritoneum, called the mesentery.  The bowels are also protected from external cold by several folds of this membrane loaded with fat.  This is known as the great omentum.