History of scientific ideas

by William Whewell

7. The view to which we are thus led, of the most promising mode of conducting the researches of Biology, is one which the greatest and most philosophical physiologists of modern times have adopted. Thus Cuvier considers this as the true office of physiology at present. ‘It belongs to modern times,’ he says, ‘to form a just classification of the vital phenomena; the task of the present time is to analyse the forces which belong to each organic element, and upon the zeal and activity which are given to this task, depends, according to my judgment, the fortune of physiology[70].’ This classification of the phenomena of life involves, of course, a distinction and arrangement of the vital functions; and the investigation of the powers by which these functions are carried on, is a natural sequel to such a classification.

[70] Hist. Sc. Nat. dep. 1789, i. 218.

8. Classification of Functions.—Attempts to classify the Vital Functions of man were made at an early period, and have been repeated in great number up to modern times. The task of classification is exposed to the same difficulties, and governed by the same conditions, in this as in other subjects. Here, as in the case of other things, there may be many classifications which are moderately good and natural, but there is only one which is the best and the true natural system. Here, as in other cases, one classification brings into view one set of relations; another, another; and each may be valuable for its special purpose. Here, as in other cases, the classes may be well constituted, though the boundary lines which divide them be somewhat indistinct, and the order doubtful. Here, [200] as in other cases, we may have approached to the natural classification without having attained it; and here, as in other cases, to define our classes is the last and hardest of our problems.

9. The most ancient classification of the Functions of living things[71], is the division of them into Vital, Natural, and Animal. The Vital Functions are those which cannot be interrupted without loss of life, as Circulation, Respiration, and Nervous Communication. The Natural Functions are those which without the intervention of the will operate on their proper occasions to preserve the bodies of animals; they are Digestion, Absorption, Nutrition; to which was added Generation. The Animal Functions are those which involve perception and will, by which the animal is distinguished from the vegetable; they are Sensibility, Locomotion, and Voice.

[71] Dict. des Sciences Nat. art. Fonctions.

The two great grounds of this division, the distinction of functions which operate continually, and those which operate occasionally; and again, the distinction of functions which involve sensation and voluntary motion from those which do not; are truly of fundamental importance, and gave a real value to this classification. It was, however, liable to obvious objections: namely, First, that the names of the classes were ill chosen; for all the functions are natural, all are vital: Second, that the lines of demarcation between the classes are indefinite and ambiguous; Respiration is a vital function, as being continually necessary to life; but it is also a natural function, since it occurs in the formation of the nutritive fluid, and an animal function, since it depends in part on the will. But these objections were not fatal, for a classification may be really sound and philosophical, though its boundary lines are vague, and its nomenclature ill selected. The division of the functions we have mentioned kept its ground long; or was employed with a subdivision of one class, so as to make them four; the vital, natural, animal and sexual functions. [201]

10. I pass over many intermediate attempts to classify the functions, and proceed to that of Bichat as that which is, I believe, the one most generally assented to in modern times. The leading principle in the scheme of this celebrated physiologist is the distinction between organic and animal life. This separation is nearly identical with the one just noticed between the vital and animal functions; but Bichat, by the contrasts which he pointed out between these classes of functions, gave a decided prominence and permanence to the distinction. The Organic Life, which in animals is analogous to the life of vegetables, and the Animal Life, which implies sensation and voluntary motion, have each its system of organs. The center of the animal life is the brain, of the organic life, the heart. The former is carried on by a symmetrical, the latter, by an unsymmetrical system of organs: the former produces intermitting, the latter continuous actions: and, in addition to these, other differences are pointed out. This distinction of the two lives, being thus established, each is subdivided into two orders of Functions. The Animal Functions are passive, as Sensation: or active, as Locomotion and Voice; again, the Organic Functions are those of Composition, which are concerned in taking matter into the system; Digestion, Absorption, Respiration, Circulation, Assimilation; and those of Decomposition, which reject the materials when they have discharged their office in the system; and these are again, Absorption, Circulation, and Secretion. To these are added Calorification, or the production of animal heat. It appears, from what has been said, that Absorption and Circulation (and we may add Assimilation and Secretion, which are difficult to separate,) belong alike to the processes of composition and decomposition; nor in truth, can we, with any rigour, separate the centripetal and centrifugal movements in that vortex which, as we shall see, is an apt image of organic life.

Several objections have been made to this classification: and in particular, to the terms thus employed. It has been asserted to be a perversion of language to [202] ascribe to animals two lives, and to call the higher faculties in man, perception and volition, the animal functions. But, as we have already said, when a classification is really good, such objections, which bear only upon the mode in which it is presented, are by no means fatal: and it is generally acknowledged by all the most philosophical cultivators of biology, that this arrangement of the functions is better suited to the purposes of the science than those which preceded it.

11. But according to the principles which we have already laid down, the solidity of such a classification is to be verified by its serving as a useful guide in biological researches. If the arrangement which we have explained be really founded in natural relations, it will be found that in proportion as physiologists have studied the separate functions above enumerated, their ideas of these functions, and of the powers by which they are carried on, have become more and more clear;—have tended more and more to the character of exact and rigorous science.

To examine how far this has been the case with regard to all the separate functions, would be to attempt to estimate the value of all the principal physiological speculations of modern times; a task far too vast and too arduous for any one to undertake who has not devoted his life to such studies. But it may properly come within the compass of our present plan to show how, with regard to the broader lines of the above classification, there has been such a progress as we have above described, from more loose and inaccurate notions of some of the vital functions to more definite and precise ideas. This I shall attempt to point out in one or two instances.