Among the modern physiologists there is no one who has more just claim to our attention than Bichat, whether we regard him as an observer of facts or as an improver of theory. In the course of a short life he acquired a very accurate and extensive knowledge of anatomy, and made many discoveries in this department of science, which seemed to have been so entirely pre-occupied by his prede cessors. In his views of the animal economy he proceed ed upon the principles of correct philosophy ; he regarded the vital functions as of a description essentially different from any other natural phenomena, and diligently applied himself to obtain an accurate knowledge of them, to ob serve their relation to each other, and to arrange them accordingly. His classification will, probably, in many of its parts, appear too refined, and his speculations to savour too much of metaphysical subtilty, but we must still consider him as possessing an unusual share of genius and acuteness, and as, perhaps, having added more to the actual stock of physiological knowledge than any of his contemporaries.
In order to obtain an insight into the true principles of physiology, we must begin by inquiring, what are the ap propriate and specific powers which distinguish the living animal from all other beings ; these we shall find to be two—spontaneous motion and sensation. Wherever spon taneous motion and sensation are found, we do not hesi tate to regard them as connected with the living body ; and we can have no idea of animal existence which does not possess one or other of these powers. These two specific properties of animal life are conceived to depend upon two principles inherent in the body, contractility, or the power of muscular contraction, and sensibility, or the power of nervous sensation; the former, the origin of motion, the latter of feeling. Haller had the merit of clearly ascertaining the nature of these two powers, and of pointing out their differences; and to him we are in debted for the development of the important fact, that they are distinguished as well by their seat as by their mode of action ; contractility being exclusively' confined to the muscular fibre, and sensibility residing only in the nervous matter. To the action of the one or other of these principles every change that is effected in the ani mal system must be referred, and it is through their im mediate operation that all the functions are performed.
Although, perhaps, in every case, they both conduce to this end, yet, as we generally perceive one to be more es sential than the other, the subjects of physiology may be divided into two classes, according as they primarily de pend upon the contractility of the muscular fibre, or the sensibility of the nervous .matter. To this division we propose to adhere in the following article ; and we shall begin with those functions which are more immediately dependent upon contractility. The functions which fall under this description are, the circulation of the blood, respiration, animal temperature, secretion, digestion, ab sorption, and generation.
But before we enter upon a description of the individual functions, it will be proper to give a more particular ac count of the nature of contractility and sensibility, and of their appropriate organs—the muscles and the nerves. For the full understanding of the subject, it be neces sary to begin by a description of membrane and bone, be cause these substances are essential ingredients of the basis of the body, and serve, as it were, for the ground-work of every other part. With respect to contractility in par ticular, they are so connected with the muscular fibre, and contribute so directly to muscular action, that, without being previously acquainted with them, it would be im possible to comprehend the functions of the muscles, or the effect of their contractions.
After going, in succession, through the different con tractile functions, we shall proceed to the other great division of the science—the functions which are more im mediately connected with the nervous power. The con nexion between the mental and corporeal parts of our frame is so intimate, that it will be impossible to acquire a complete knowledge of the one without paying some attention to the other ; and, therefore, although we shall be anxious to encroach, as little as possible, upon the province of the metaphysician, we shall feel it necessary to take a brief view of some of the intellectual operations, as well as of those that depend upon different modifications of the power of sensation. We shall conclude by some observations upon the natural progress which the living system manifests to a state of dissolution, by which its component parts necessarily fall into decay, and its func tions become impaired and finally destroyed.