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Show 68 Tiim PRtNC1PtE or TH£ DIRECT CHAP. III. heard of an exactly su· nt·1 at. case ' vith an A. ustralia. n . h d b n lent. :B·\ne mus1c, ~:ll:tvao·e to wbotn a gun a ee . fromo th' e vu,o·ue emotw. ns thus exm' te d ' cau ses a shiver to run dow~ the backs of some persons. There seetns to be very little in common in the above sev~ral. phyt: iical causes and enlotions to account for trembling' and ~. J Paaet to whom I am indebted for several of the 1-Jlr . 0 ' b' . . above statements, informs n1e that the ~u ~ect IS a vel Y t> bs.c ur~ e one. As tremblin ocr is son1etimes. caused b.y racre, long before exhaustion can have set 1n, and as It so~etimes accompanies great joy, it would a~pear that any strong excitement of the nervous systen~ Interrupts the steady flow of nerve-force to the muscles. . The manner in which the secretions of the alunentary canal and of certain glands-as the li:er, k.idneys, or mammm----are affected by strong e1notions, IS ano~her excellent instance of the direct action of the sensonun1 on these organs, independently of th~ will or of a~y serviceable associated habit. There is the greatest difference in different persons in the parts which are thus nffected and in the decrree of their affection. ' 0 b . The heart, which goes on uninterruptedly eating night and day in so wonderful a manner, is ex~ren1~ly sensitive to external stitnulants. The great physiologist, Claude Bernard,3 has shown how the least excitement of a sensitive nerve reacts on the heart; even when a nerve is touched so slightly that no pain can possibly be felt by the animal under experiment. Hence whe.n the 1nind is strongly excited, we might expect that It would instantly affect in a direct n1anner the heart; 2 Miiller remnrks ('Elements of Physiology,' Eng. translat. vol. ii. p. !lil4) that when the fr.elings are very intense, "all the spinal. nerves H bocomo n.ffectod to the extent of imperfect paralysis, or the exCttemullt " of trembling of the whole body." 3 'Le<;ons sur los Prop. des 'fissus Vivants,' 1860, PP• 457-466, CrrAP. III. ACTION OF THE NERVOUS SYSTEM. 69 and this is universally acknowledged and felt to be the case. Claude Bernard also repeatedly in ists, and this deserves especial notice, that when the heart is affected it reacts on the brain; and the state of the brain acrain reacts through the pneumo-gastric nerve on the he~.rt ; so that under any excitement there will be much mu .. tual action and reaction between these, the two n1ost i1nportaut organs of the body. 1,he vaso-motor system, which regulates the diameter of the small arteries, is directly acted on by the sensorium, as we see when a man blushes from shame· but in this latter case the ehecked transmiss~on of ne' rve-force to the vessels of the faee can, I think, be pa1'tly explained in a curious manner through habit. We shall also be able to throw some light, though very little, on the involuntary erection of the hair under the mnotions of terror and rage. ~rhe secretion of tears clepends, no doubt, on the connection of certain nervece! Is; but here again we can trace some few of the steps by which the flow of nerve-force through the requisite channels has becmne habitual under certain emotions. A brief consideration of the outward signs of some of the stronger sensations and e1notions will best serve to show us, although vaguely, in how co1nplex a manner the principle under consideration of the direct action of the excited nervous system on the body, is combined with the principle of habitually associated, serviceable movements. When animals suffer from an agony of pain, they generally writhe about with friO'htful contortions· and b ' those which habitually use their voices utter piercing cries or groans. Almost every 1nuscle of the body is brought into strong action. 'Vith man the n1outh m[l.y be closely compressed, or more commonly the |
