verable by any external signs; on which account it is difficult, and in many cafes impoffible, to foresee the effects of applications that may be made to the body; which renders the art of medicine in a great degree conjectural*.

7. As the fufceptibility of the living body varies in the manner now mentioned, fo likewise all the powers which are capable of influencing it, may be faid, in fome fort, to act fpecifically; fince no two of them produce precifely fimilar effects. This appears to be equally true of the ordinary healthful agents, and of those which induce or remove diseases.

8. Every medicine poffeffes properties more or lefs peculiar to it, which determine its action to fome particular part or organ, in preference to others. Thus cantharides ftimulate the bladder and neighbouring parts; ipecacuanha, the ftomach; and aloes, the inteftines. Spices and the like, that frequently prove fo falutary to the ftomach, painfully irritate the eye or the fur

* Medicina ars conjecturalis: this maxim was not more true in the days of Celfus, than it is at prefent; in spite of the boasted improvements in phyfiology and practice of later times.

face of a wound. Opium, henbane, digitalis, and various other narcotics, whether poisonous or medicinal, immediately disturb the functions of the brain and nerves; while the foffil poifons, as lead, arfenic, and barytes, excite other parts to morbid action, but exert no direct influence on the brain.

9. A confiderable variety exists even among the individuals of the same clafs of medicines. We obferve, for inftance, a wide difference between the effects of different purgatives; fo that one cannot always be fubftituted for another in practice, without disadvantage, though agreeing in their common operation. And the fame is true of the other claffes of the Materia Medica.

10. This applies alfo to morbific agents, and particularly to morbid poifons, each of which feems to have its peculiar feat in the body. The variolous poifon limits its action chiefly to the skin, rarely affecting the internal parts; while catarrhal inflammation is excited by the morbillous poison; and fore throat by that of fcarlatina. Thus, too, the venereal poison affects certain bones almoft exclufively.

11. In like manner, the paffions of the mind produce effects on the body, each in

a great measure peculiar to itself. Thus fear produces contraction of the extreme veffels, and of the mufcular coat of the bladder; while it debilitates the muscles of voluntary motion, or at leaft diminishes the influence of the will over them. Shame excites the veffels of the cheeks to unusual action; and fo of the rest.In a word, although there may be powers which prove ftimulant to any individual part of the fyftem to which they are applied (fuch, for example, as heat), it may be doubted whether, by any mode of application, they can be made to produce an univerfal and direct effect.

12. A relation fubfifts between the different parts of the fyftem, in confequence of which any confiderable change in the condition of one part, affects, more or less, the action of others. This relation, which is termed fympathy or confent, cannot be explained by a community of nerves or veffels between the parts thus influencing each other*, but appears to depend on the brain,

* The pulmonic plexus which furnishes the lungs with nerves, has no greater communication with the phrenics, and thote that fupply the intercostal muscles, than with the nerves of the ftomach, inteftines, or other abdominal vifcera; nor do thofe nerves arife from the fame part of the

and its continuation the fpinal marrow; for thefe being destroyed, fympathy no longer exifts*.

13. The parts of the body differ greatly from one another, in the readiness with which they influence, or are influenced by, others; fome fympathizing with almost every part of the fyftem, whilft others often undergo great changes, without affecting the reft of the body as examples of the former, may be mentioned the brain and ftomach; of the latter, the common cellular membrane, and various glandular parts.

14. The more readily and extensively any part fympathizes with the rest of the fyftem, the greater are the number and va riety of the symptoms which accompany its difeafed ftate,

15. Sympathetic affections have no neceffary analogy with the actions which pro

brain. The fympathy, therefore, which exifts fo remarkably between the lungs and the muscles of respiration, and in confequence of which any irritation of the former excites the latter to immediate action, does not depend on any direct nervous connection. The fame is true of the fenfitive and moving parts of the eye in refpect of one another, the nerves of which are distinct, as well in their origin as their diftribution. Many other fimilar inftances in the fyftem might be adduced.

* Whytt's Works, 4to, p. 510.

duced them, but are often of the reverse kind. Inflammation of the kidney, though it affects the ftomach, does not excite any thing like inflammation in it.

16. It seems to be a general law of the animal œconomy, that the action of one part being preternaturally increased, the reft of the fyftem, or particular parts of it, have their natural actions diminished; and vice verfa. Thus an increase of the fecretion and peristaltic motion of the intestines, as in diarrhoea, is attended with impaired action of the ftomach, and torpor of the veffels of the fkin; while there is no more certain method of diminishing exceffive action in the alimentary canal, than increasing the circulation on the furface of the body. On the other hand, by reducing the action of the veffels of the fkin, as by the moderate application of cold, the functions of the ftomach are often performed with augmented vigour.

17. An intimate acquaintance with the fympathies that fubfift between the different parts of the fyftem, is of the greatest importance to the medical practitioner, in enabling him to detect the primary feats of difeases; as will be feen more particularly hereafter.