Under the thinking or mental organs are classed, first, the organs of sense, where impressions take place; second, the cerebrum or organ of thought, properly so called, where these excite ideas, emotions, and passions; and third, the cerebel or organ of volition, where acts of the will result from the last.[24]
We may now more particularly notice the functions of these organs, which are the subject of physiology.
In the locomotive functions, the bones at once give support, and form levers for motion; the ligaments form articulations, and afford the points of support; and the muscles are the moving powers. To the first, are owing all the symmetry and elegance of human form; to the second, its beautiful flexibility; and to the third, all the brilliance and grace of motion which fancy can inspire, or skill can execute.
In the nutritive functions, the food, having passed into the mouth, is, after mastication, aided by mixture with the saliva, thrown back, by the tongue and contiguous parts, into the cavity behind, called fauces and pharynx; this contracting, presses it into the œsophagus or gullet; this also contracting, propels it into the stomach, which, after its due digestion aided by the gastric juice, similarly contracting, transmits whatever portion of it, now called chyme, is sufficiently comminuted to pass through its lower opening, the pylorus, into the intestines; these, at the commencement of which it receives the bile and pancreatic juice, similarly pressing it on all sides, urge forward its most solid part to the anus; while its liquid portion partly escapes from the pressure into the mouths of the absorbents. The absorbents arising by minute openings from all the internal surfaces, and continuing a similar contractile motion, transmit it, now called chyle, by all their gradually-enlarging branches, and through their general trunk, the thoracic duct, where it is blended with the lymph brought from other parts, into the great veins contiguous to the heart, where it is mixed with the venous or returning and dark-colored blood, and whence it flows into the anterior side of that organ. The anterior side of the heart, forcibly repeating this contraction, propels it, commixed with the venous blood, into the lungs, which perform the office of respiration, and in some measure of sanguification; there, giving off carbonaceous matter, and assuming a vermilion hue and new vivifying properties, it flows back as arterial blood, into the posterior side of the heart. The posterior side of the heart, still similarly contracting, discharges it into the arteries; these, maintaining a like contraction, carry it over all the system; and a great portion of it, impregnated with carbon, and of a dark color, returns through the veins in order to undergo the same course. Much, however, of its gelatinous and fibrous parts is retained in the cells of the parenchyma, or cellular, vascular, and nervous substance forming the basis of the whole fabric, and constitutes nutrition, properly so called; while other portions of it become entangled in the peculiarly-formed labyrinths of the glands, and form secretion and excretion—the products of the former contributing to the exercise of other functions, and those of the latter being rejected. As digestion precedes the first, so generation follows the last of these functions, and not only continues the same species of action, but propagates it widely to new existences in the manner just described.
In the thinking functions, the organs of sense receive external impressions, which excite in them sensations; the cerebrum, having these transmitted to it, performs the more complicated functions of mental operation, whence result ideas, emotions, and passions; and the cerebel, being similarly influenced, performs the function of volition, or causes the acts of the will.
It is not unusual to consider the body as being divided into the head, the trunk, and the extremities; but, in consequence of the hitherto universal neglect of the natural arrangement of the organs and functions into locomotive, nutritive, and thinking, the beauty and interest which may be attached to this division, have equally escaped the notice of anatomists.
It is a curious fact, and strongly confirmative of the preceding arrangements, that one of these parts, the extremities, consists almost entirely of locomotive organs, namely, of bones, ligaments, and muscles; that another, the trunk, consists of all the greater nutritive organs, namely, absorbents, bloodvessels, and glands; and that the third, the head, contains all the thinking organs, namely, the organs of sense, cerebrum, and cerebel.[25]
It is a fact not less curious, nor less confirmative of the preceding arrangements, that, of these parts, those which consist chiefly of locomotive or mechanical organs—organs which, as to mere structure, and considered apart from the influence of the nervous system over them, are common to us with the lowest class of beings, namely, minerals[26]—are placed in the lowest situation, namely, the extremities; that which consists chiefly of nutritive or vital organs—organs common to us with a higher class of beings, namely, vegetables[27]—is placed in a higher situation, namely, the trunk; and that which consists chiefly of thinking or mental organs—organs peculiar to the highest class of beings, namely, animals[28]—is placed in the highest situation, namely, the head.
It is not less remarkable, that this analogy is supported even in its minutest details; for, to choose the nutritive organs contained in the trunk as an illustration, it is a fact, that those of absorption and secretion, which are most common to us with plants, a lower class of beings, have a lower situation—in the cavity of the abdomen; while those of circulation, which are very imperfect in plants,[29] and more peculiar to animals, a higher class of beings, hold a higher situation—in the cavity of the thorax.
It is, moreover, worthy of remark, and still illustrative of the preceding arrangements, that, in each of these three situations, the bones differ both in, position and in form. In the extremities, they are situated internally to the soft parts, and are generally of cylindrical form; in the trunk, they begin to assume a more external situation and a flatter form, because they protect nutritive and more important parts, which they do not, however, altogether cover; and, in the head, they obtain the most external situation and the flattest form, especially in its highest part, because they protect thinking and most important organs, which, in some parts, they completely invest.