regarded as not at all different in any respect from the ordinary chemical changes taking place outside the body. This, however, is not only erroneous in theory, but conduces also to a vicious mode of study. For it draws away our attention from the phenomena themselves and their real characteristics, and leads us to deduce one set of phenomena from what we know of another; a method which we have already shown to be unsafe and pernicious. It has also been asserted that the phenomena of the nervous system are identical with those of electricity; for no other reason than that there exist between them certain general resemblances. But when we examine the phenomena in detail, we find that, beside these general resemblances, there are many essential points of dissimilarity, which must be suppressed and kept out of sight in order to sustain the idea of the assumed identity. This assumption is consequently a forced and unnatural one, and the simplicity which it was intended to introduce into our physiological theories is imaginary and deceptive, and is attained only by sacrificing a part of those scientific truths, which are alone the real object of our study. We should avoid, therefore, making any such unfounded comparisons; for the theoretical simplicity which results from them does not compensate for the loss of essential scientific details.

VI. The study of Physiology is naturally divided into three distinct Sections :

The first of these includes everything which relates to the NUTRITION of the body in its widest sense. It comprises the history of the proximate principles, their source, the manner of their production, the proportions in which they exist in different kinds of food and drink, the processes of digestion and absorption, and the constitution of the circulating fluids; then the physical phenomena of the circulation and the forces by which it is accomplished; the changes which the blood undergoes in different parts of the body; all the phenomena, both physical and chemical, of respiration; those of secretion and excretion, and the character and destination of the secreted and excreted fluids. All these processes have reference to a common object, viz., the preservation of the internal structure and healthy organization of the individual. With certain modifications, they take place in vegetables as well as in animals, and are consequently known by the name of the vegetative functions.

The Second Section, in the natural order of study, is devoted to the phenomena of the NERVOUS SYSTEM. These phenomena are

not exhibited by vegetables, but belong exclusively to animal organizations. They bring the animal body into relation with the external world, and preserve it from external dangers, through the means of sensation, movement, consciousness, and volition. They are more particularly distinguished by the name of the animal functions.

Lastly comes the study of the entire process of REPRODUCTION. Its phenomena, again, with certain modifications, are met with in both animals and vegetables; and might, therefore, with some propriety, be included under the head of vegetative functions. But their distinguishing peculiarity is, that they have for their object the production of new organisms, which take the place of the old and remain after they have disappeared. These phenomena do not, therefore, relate to the preservation of the individual, but to that of the species; and any study which concerns the species comes properly after we have finished everything relating to the individual.

SECTION I.

NUTRITION.

CHAPTER I.

PROXIMATE PRINCIPLES IN GENERAL.

THE study of NUTRITION begins naturally with that of the proximate principles, or the substances entering into the composition of the different parts of the body, and the different kinds of food. In examining the body, the anatomist finds that it is composed, first, of various parts, which are easily recognized by the eye, and which occupy distinct situations. In the case of the human body, for example, a division is easily made of the entire frame into the head, the neck, the trunk, and extremities. Each of these regions, again, is found, on examination, to contain several distinct parts, or "organs," which require to be separated from each other by dissection, and which are distinguished by their form, color, texture, and consistency. In a single limb, for example, every bone and every muscle constitutes a distinct organ. In the trunk, we have the heart, the lungs, the liver, spleen, kidneys, spinal cord, &c., each of which is also a distinct organ. When a number of organs, differing in size and form, but similar in texture, are found scattered throughout the entire frame, or a large portion of it, they form a connected set or order of parts, which is called a "system." Thus, all the muscles taken together constitute the muscular system; all the bones, the osseous system; all the arteries, the arterial system. Several entirely different organs may also be connected with each other, so that their associated actions may tend to accomplish a single object, and they then form an "apparatus." Thus the heart, arteries, capillaries, and veins, together, form the circulatory apparatus; the stomach, liver, pancreas, intestine, &c., the digestive apparatus. Every organ, again, on microscopic examination, is

seen to be made up of minute bodies, of definite size and figure, which are so small as to be invisible to the naked eye, and which, after separation from each other, cannot be further subdivided without destroying their organization. They are, therefore, called “anatomical elements." Thus, in the liver, there are hepatic cells, capillary blood vessels, the fibres of Glisson's capsule, and the ultimate filaments of the hepatic nerves. Lastly, two or more kinds of anatomical elements, interwoven with each other in a particular manner, form a "tissue." Adipose vesicles, with capillaries and nerve tubes, form adipose tissue. White fibres and elastic fibres, with capillaries and nerve tubes, form areolar tissue. Thus the solid parts of the entire body are made up of anatomical elements, tissues, organs, systems, and apparatuses. Every organized frame, and even every apparatus, every organ, and every tissue, is made up of different parts, variously interwoven and connected with each other, and it is this character which constitutes its organization.

But besides the above solid forms, there are also certain fluids, which are constantly present in various parts of the body, and which, from their peculiar constitution, are termed "animal fluids." These fluids are just as much an essential part of the body as the solids. The blood and the lymph, for example, the pericardial and synovial fluids, the saliva, which always exists more or less abundantly in the ducts of the parotid gland, the bile in the biliary ducts and the gall-bladder: all these go to make up the entire body, and are quite as necessary to its structure as the muscles or the nerves. Now, if these fluids be examined, they are found to be made up of many different substances, which are mingled together in certain propor tions; these proportions being constantly maintained at or about the same standard by the natural processes of nutrition. Such a fluid is termed an organized fluid. It is organized by virtue of the numerous ingredients which enter into its composition, and the regular proportions in which these ingredients are maintained. Thus, in the plasma of the blood, we have albumen, fibrin, water, chlorides, carbonates, phosphates, &c. In the urine, we find water, urea, urate of soda, creatine, creatinine, coloring matter, salts, &c. These substances, which are mingled together so as to make up, in each instance, by their intimate union, a homogeneous liquid, are called the PROXIMATE PRINCIPLES of the animal fluid.

In the solids, however, even in those parts which are apparently homogeneous, there is the same mixture of different ingredients. In the hard substance of bone, for example, there is, first, water,