Licence: Public Domain Mark
Credit: A text book of physiology / by William Rutherford. Source: Wellcome Collection.
Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.
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![action of the gastric juice upon the food is a purely cliemical problem. But there ai'e phenomena within the body that are restricted to living things, which have laws of their own, and are not exjilained in the present state of our knowledge by the laws of chemistry or physics—e.g., develop- ment, nutrition, sensation, volition, etc. These are termed vital, because they are pcculitir to living beings. Yet the history of physiology clearly shows that real progress has been made only when the accepted theories regarding vital phenomena have been in perfect harmony with,physico- chemical laws. Much knowledge of the bodily functions has been attained (1) by merely observing vital phenomena in their normal condition. (2) Much more, however, has been learned by experiments on animals and on man; for the method of experimentation is far more powerful than that of mere observation in revealing function. Although there are points of specific difference between man and other vertebrates, there are so many points of strict resemblance, that inferences from the results of many physiological experiments performed on dogs, rabbits, frogs, and other animals, are applicable to the case of man. The principal physiological difference between man and the vertebrates immediately below him is to be found in the more highly specialised character of brain action, yet even the physiology of the human brain has been gi'eatly advanced by experiments on the brains of animals. There are other minor points of difference, but it cannot be doubted that the j^hysiology of bone, cartilage, muscle, nerve, and other tissues in a dog or rabbit is essentially the same as in man. Moreover, the heart, blood and lymph vessels, the lungs and kidneys, the liver, sali\'ary glands, stomach, and intestines of a dog or a rabbit discharge functions essentially similar to those observed in man. But it must be admitted that there are various differences in points of detail here and there observable; so that although experiments on animals have been of infinite value in human physiology since the time when Galen proved that the arteries during life contain blood, and Harvey demonstrated the circulation of that fluid, nevertheless, in no case can the result of an experi- ment on an animal be regarded as more than an index of what may he ex]}ected to hold true in man. It is merely presumptive evidence until its truth is directly proved in his case. Yet so highly is this presumptive evidence valued, that no one would dare to test the effect of some new remedy upon a human being without first of all experimenting with it on animals; for although the effects in the two cases often differ in degree, they are most commonly similar in kind. (3) Physiological knowledge is also arrived at by studying the phenomena of disease. Disease may be generally defined as an abnormal physiological condition; in other words, pathology is a modified physiology. Disease works experiments often so refined that they cannot be imitated by art. Thus, from the observa- tion that in some cases of enlargement of the spleen the blood contains an increased number of white corpuscles, the novel conclusion was arrived at that a function of the spleen is to produce white blood-corpuscles. Many other facts in physiology, especially in that of the nervous system, have been derived from this important field of inquiry.](https://iiif.wellcomecollection.org/image/b21981747_0016.jp2/full/800%2C/0/default.jpg)
