Their elaborate Machinery. - Their Structure varies according to the Food of the Animal to which they belong. - Enumeration of the several digestive Organs. - Their extraordinary sympathetic relations. - The alimentary Canal: - its peristaltic Motion. - The Stomach: - its Figure, Dimensions, Situation, and Structure. - Small Intestines. - The Duodenum: - Peculiarities of its Functions entitle it to be considered as a second Stomach: - Provisions to limit its Motions. - Jejunum. - Ilium. - Large Intestines. - Csecum. - Colon. - Rectum. - The various Glands, or secreting Organs, for the Preparation of the Digestive Fluids. - The salivary Glands. - Glands of the Stomach and Intestines. - -The Liver. - The Pancreas. - Observations on the supposed Use of the Spleen. - Vessels for carrying the nutritive Product to the current of the Circulation. - The Lacteals. - Mesenteric Glands. - The Thoracic Duct. - The Lungs. - The Kidneys. - The Skin.

10. No function in the animal economy presents such elaborate machinery as that of digestion; but its complexity and extent have been found to vary according to the nature of the food upon which it is designed to act. If it greatly differ in composition from the matter of which the animal is constituted, the changes it has to undergo before it can be adapted for the support and reparation of the body which receives it, must necessarily be more considerable, and the organs are accordingly more extensive and elaborate in herbivorous than in carnivorous animals: while man, who derives his supplies of nourishment from both the kingdoms of nature, possesses an intermediate organization. His digestive organs may be said to consist of:

1. A long canal, extending from the mouth to the anus, varying in the diameter of its different parts, according to the distinct duties which each is destined to perform; and which are also capable of contracting or enlarging their dimensions according to the circumstances under which they act.

2. Various glands, or secreting organs, for the preparation of the liquids which are required for acting on the alimentary matters.

3. Vessels for conveying to the current of the circulation the nutritive product of the operation.

4. The lungs, which complete its assimilation with the blood.

5. The kidneys and skin, which carry off the remaining portion as excrementitious.

These different organs are not only intimately connected with each other, but they display an extraordinary sympathetic relation with the sanguiferous and cerebral systems: there is, for instance, no organ of the body which is not directly or indirectly affected by the operations of the stomach: we shall therefore cease to wonder that an impression made upon it by a medicinal agent, or by an alimentary substance, should afford the means of exciting an action in the most distant parts of the machine; nor can we be surprised that the aberrations of this central organ should give origin to the greater number of maladies with which the body is afflicted; nor that those applications should be so effective, which are directed, for their cure, through the medium of its sympathies. But that we may not, like the members in the ancient fable, wage an unjust war against the stomach for the maladies which it may thus inflict, it is necessary to state, that the stomach suffers equally, in its turn, from the derangement of distant organs.

What practitioner has not witnessed the sudden sickness produced by the sprains of tendinous and ligamentous structures, or by blows on the head or other parts? To distinguish between the sympathetic and primary affections of the digestive organs, is a problem of the greatest practical importance; and the profession is much indebted to Mr. Abernethy, for his endeavours to show how the stomach and bowels may become affected from local disorders, and vice versa.

11. Although it would be obviously foreign to the plan and objects of this work to enter into minute anatomical investigations, yet as there are certain facts, connected with the structure and locality of the alimentary organs, which it is essential for the practitioner to bear in mind, I shall here offer such a description of them as may appear necessary for bis guidance, or for the maintenance of that perspicuity which I am anxious to bestow upon the following pages. In the performance of this task, I shall preserve the order of arrangement already noticed (10), viz.