It would be very interesting to discover by what resources Nature thus effects the production of the same kind of nutritive fluid or _chyle_, from so great a variety of substances, and apportions to every part the precise elements of which it stands in need; but it is doubtful whether the human faculties were ever designed to penetrate so far into the modes of vital action, and, in the mean time, it will be better for us to confine our attention to that branch of the inquiry which we can easily comprehend, and which bears a direct reference to our own welfare. We know already that certain organs are concerned in the processes above mentioned, and that these organs act under the regulation of certain general laws. If we make ourselves acquainted with, and carefully fulfil, these laws, we reap a rich reward in the enjoyment of sound and vigorous digestion. Whereas, if, either from ignorance or from carelessness, we neglect their fulfilment, we bring upon ourselves a severe punishment in the form of dyspeptic or nervous disease. Assuredly, then, alternatives like these ought to excite some desire for information in the minds even of the most indifferent. * * * * * Before, however, commencing a description of the organs concerned in digestion, it will be useful to take a general view of the different stages of preparation through which the food passes, between its reception into the stomach and its _assimilation_, or ultimate conversion into a constituent part of the animal body, and becoming endowed with the properties of life. The reader will thus be better enabled to understand the meaning of various terms and expressions, the frequent use of which it is almost impossible to avoid, even in the beginning of our exposition.