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rately to the left, bcth in size and shape; hut the arterial branches, which supply the two arms, do not go off from their trunk, in a pair, in the same manner, at the fame place, or at the fame angle. Under which want of similitude, it is very difficult to conceive how the fame quantity of blood should be pushed through each artery: yet the result is right; the two limbs, which are nourished by them, perceive no difference of supply, no effects of excess or deficiency.
Concerning the difference of manner, in which the subclavian and carotid arteries, upon the different sides of the body, separate themselves from the aorta, Cheselden seems to have thought, that the advantage which the left gain by going off at a much acuter angle than the right, is made up to the right by their going off together in one branch', it is'very possible that this may be the compenfating contrivance: and, if it be so, how curious, how hydrostatical!
II. Another persection of the animal mass is the package. I know nothing which is so surprising. Examine the contents of the
* Chef. Aaat. p. 184. cd. 7.
trunk trunk of any large animal. Take notice how soft, how tender, how intricate they are; how constantly in action, how necessary to life. Reflect upon the danger of any injury to their substance, any derangement of their position, any obstruction to their office. Observe the heart pumping at the centre, at the rate of eighty strokes in a miuute: one set of pipes carrying the stream away from it, another set, bringing, in its course, the fluid back to it again : the lungs performing their elaborate office, viz. distending and contracting their many thoufand vesicles, by a reciprocation which cannot cease for a minute: the stomach exercising its powerful chemistry: the bowels silently propelling the changed aliment ; collecting from it, as it proceeds, and transmitting to the blood an incessant supply of prepared and assimilated nourishment: that blood pursuing its course; the liver, the kidneys, the pancreas, the parotid, with many other known and distinguishable glands, drawing off from it, all the while, their proper secretions. These several operations, together with others more subtile but less capable of being investigated, are going orl within us, at one and the fame time. Think of this; and then observe how the body itself, the case which holds this machinery, is rolled, and jolted, and tossed about, the mechanism remaining unhurt, and with very little molestation even of its nicest motions. Observe a rope dancer, a tumbler, or a monkey ; the sudden inversions and contortions which the internal parts sustain by the postures into which their bodies are thrown; or rather observe the shocks, which these parts, even in ordinary subjects, sometimes receive from falls and bruises, or by abrupt jerks and twists, without sensible, or with soon recovered damage. Observe this, and then reflect how firmly every part must be secured, how carefully surrounded, how well tied down and packed together.
This property of animal bodies has never, I think, been considered under a distinct head, -or so fully as it deserves, I may be allowed therefore, in order to verify my observation concerning it, to set forth a short anatomical detail, though it oblige me to use more technical language, than I should wish to introduce into a work of this kind.
i. The heart (such care is taken of the
centre centre of life) is placed between two soft lobes of the lungs ; is tied to the mediastinum and to the pericardium, which pericardium is not only itself an exceedingly strong membrane,but adheres firmly to the duplicature of the mediastinum, and, by its point, to the middle tendon of the diaphragm. The heart is also Jhjlained in its place by the great bloodvessels which issue from it'.
2. The lungs are tied to the sternum by the mediastinum, before; to the vertebrae by the pleura, behind. It seems indeed to be the very use of the mediastinum (which is a membrane that goes straight through the middle of the thorax, from the breast to the back) to keep the contents of the thorax in their'places; ia particular to hinder one lobe of the lungs from incommoding another, or the parts of the lungs from pressing upon each other when we lie on one side f.
3. The liver is fastened in the body by two ligaments; the first, which is large and strong, comes from the covering of the diaphragm, and penetrates the substance of the liver; the second is the umbilical vein, which, after
* Kcill's Anat. p. 107. cd. 3. t H>. 119.
birth, degenerates into a ligament. The first, which is the principal, fixes the liver in its situation, whilst the body holds an erect posture; the second prevents it from pressing upon the diaphragm when we lie down; and both together fling or suspend the liver when we lie upon our backs, so that it may not compress or obstruct the ascending vena cava*, to which belongs the important office of returning the blood from the body to the heart.
4. The bladder is tied to the navel by the urachus transformed into a ligament: thus, what was a passage for urine to the fetus becomes, after birth, a support or stay to the bladder. The peritonæum also keeps the viscera from confounding themselves with, or pressing irregularly upon, the bladder: for the kidneys and bladder are contained in a distinct duplicature of that membrane, being thereby partitioned off from the other contents of the abdomen.
5. The kidneys are lodged in a .bed of fat.
6. The pancreas or sweetbread is strongly tied to the peritoneum, which is the great
• Chef. Anat. p. 162.