56 OUR OPINIONS AND PRINCIPLES. Such truths we regard as the first principles on which the superstructure of man's knowledge rests. When this acknowledgment is made, we may embark on the wide ocean of physical investigation, without fear of reaching those impious conclusions to which we have above referred. When we add, that every proper occasion will be seized to develope the true grounds on which Teleology rests, without at all infringing upon the precepts of Bacon with regard to the possible abuse of final causes in philosophical investigations, we think we have sufficiently indicated the character which this work will sustain as respects OPINIONS AND PRINCIPLES. Order in Physiology.-The Physiology of Animal and Vegetable Life, being a subject of great extent, must be methodically treated; and first, it is necessary to determine what principle of arrangement is to be adopted, in order to exhibit, in a connected form, the complete phenomena of these kinds of existence. There are several modes in which such phenomena have been methodized; and it will be convenient briefly to consider some of these, as exhibiting a general view of the whole subject. The phenomena of animal and vegetable life may be described as Mechanical Phenomena, Chemical Phenomena, Electrical Phenomena, and the peculiar Phenomena of Excitability-the first three orders being common to all departments of nature. A great part of many of the most important actions of the perfect animal body are purely mechanical or purely chemical, or partly chemical or partly mechanical; while such actions are, in their remaining part, the result of a peculiar excitability. In the circulation of the blood, for example, in man, and in the animals resembling man, the blood is propelled onwards by mechanical forces, while these mechanical forces are called into activity in obedience to the laws of excitability. In the function of respiration the air enters the lungs in conformity with the laws of that part of mechanical science termed Pneumatics. The change which the blood undergoes by the contact of this air is a chemical change, or a change closely analogous to a chemical change; while these laws of pneumatics, and the chemical laws, are brought into operation by the agency of an organic excitability. The fluids contained in the leaves of plants in contact with atmospheric air, by the influence of light, undergo a chemical change, or a change exactly 60 VEGETATIVE FUNCTIONS. examination teaches that this is not the case. Vegetables possess no tissues which allow of the same kind of nutritive absorption, of distribution of juices, or of secretion, that we meet with in, at least, the higher animals. They have no large cavities in which considerable quantities of food can be collected and dissolved by special fluid secretions. They possess no point midway in the movement of their juices, and no mechanism, other than that of a casual and secondary apparatus for the inhaustion or the expulsion of the respiratory gases. They are devoid of the changeable epithelial coverings, which play an important part in many of the animal excretory organs. In one word, the general organic functions are introduced into the two living kingdoms of nature, and probably into their subordinate divisions, by two different ways. This difference leads at once to the conclusion, that the structure of the animal is not a simple repetition of that of the plant, with the addition of a series of new apparatus. The nature of the tissues, the mode of their actions and change form, division, and destiny of the organs- all these rather teach us that animals of any development are constructed upon an altogether different plan." Whatever in the above quotation may appear obscure to those to whom physiological ideas are new, will be cleared up, we trust, by what we are about to say on the prominent distinctions between those organic existences which are unequivocally animals, and those which are unequivocally plants, with reference to a basis for the arrangement of the phenomena of vegetable and animal life. In physiology, the term function is of continual occurrence. What, then, does function signify? Function is the use of a part or organ. The function of the eye is sight; that of the ear, hearing; that of the lungs, the purification of the blood by ventilation; that of the stomach, digestion; that of the liver, to secrete bile. In plantsthat of the spongioles of the radicles, to absorb from the soil; that of the leaves, to decompose the carbonic acid of the atmosphere, so as to appropriate the carbon for the uses of the plant; that of the anther, to impregnate the ovule, by means of its secretion, the pollen; that of the ovary, to mature the ovule into a seed. As the functions in all the higher animals and the higher plants. are numerous, there is room for method in the arrangement of them. Various methods have been suggested; and, in accordance with some one or other of these arrangements, it has been common to methodize the various topics belonging to physiology. The kinds of function common to plants and animals, are properly termed vegetative functions-the same which are called vegetable or general organic functions in the quotation from Valentin. The kinds of function, not so obviously possessed by plants, so as to seem peculiar to animals, are named the animal functions. VEGETATIVE FUNCTIONS. ce; 61 the The vegetative functions are the functions of maintenar animal functions are the relative functions, or the functions of rela tion. The vegetative func tions end in the organism of the individual, or, at most, in the organism of the species; the functions of relation establish relations between the animal and the world without. If we follow the food, in one of the higher animals, from the mouth to its incorporation with the previously existing tissues of the body, the waste of which it is its office to supply, we shall discover what are the more immediate vegetative functions the same which, by other names, are known as the functions of maintenance; the functions of nutrition; the assimilative functions, or functions of assimilation; and the functions of organic life. The food - let it be a piece of meat, or bread is reduced to a pulp by the movements of the teeth, and the admixture of the saliva, secreted by the salivary glands; it is then swallowed by a somewhat complex muscular action. It is moved about in the stomach by the contraction of its muscular fibres; and, being mixed with the gastric juice, a peculiar fluid secreted by the lining membrane of the stomach, it passes into chyme: this chyme is then, in successive portions, transmitted, by muscular contraction, into the highest part of the intestinal tube, termed the duodenum, which is a kind of second stomach, where the partially assimilated food is first mixed with the bile, and then with the secretion derived from the sweetbread, or pancreas. The mass is now ready to afford chyle, the immediate nourishment of the blood, to the absorbent 62 ASSIMILATIVE ORGANS. vessels, termed lacteals, the extremities of which abut on the lining membrane of the higher parts of the intestinal tube, while the residue is sent downwards by what is termed the peristaltic action of the tube, for evacuation. The chyle, taken up on a very wonderful plan by the lacteal tubes, is transmitted through the singular small organs a, thoracic duct receiving lacteal tubes from b, the intestine; c, aorta. termed the mesenteric glands, whence, after important changes, it is again collected by what are named the efferent lacteal tubes; these by degrees unite together into a trunk, which joins the lymphathic vessels coming from the pelvis and the lower parts of the body, to form the thoracic duct a, commencing in the abdomen, dividing opposite the middle of the dorsal vertebræ into two branches, which soon reunite, passing behind the arch of the aorta and subcla10, the greater vena azygos, in vian artery, and making its turn at b, which, in some mammals, the duct where it receives several lymphatic trunks, terminates at the point of junction of the internal jugular and subclavian veins on the left side of the neck, and into which it pours its contents. The chyle, being thus mixed with the venous blood, is carried with it to the right side THE COURSE AND TERMINATION OF THE terminates. |