a history of science-4-第50章
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fferent order and obey totally different laws； being excited by causes in a situation which is EXCENTRIC in the nervous systemthat is； distant from the nervous centres。 This mode of action has not； I think； been hitherto distinctly understood by physiologists。
〃Many of the phenomena of this principle of action； as they occur in the limbs； have certainly been observed。 But； in the first place； this function is by no means confined to the limbs； for； while it imparts to each muscle its appropriate tone； and to each system of muscles its appropriate equilibrium or balance； it performs the still more important office of presiding over the orifices and terminations of each of the internal canals in the animal economy； giving them their due form and action； and； in the second place； in the instances in which the phenomena of this function have been noticed； they have been confounded； as I have stated； with those of sensation and volition； or； if they have been distinguished from these； they have been too indefinitely denominated instinctive； or automatic。 I have been compelled； therefore； to adopt some new designation for them； and I shall now give the reasons for my choice of that which is given in the title of this paper'Reflex Functions。'
〃This property is characterized by being EXCITED in its action and REFLEX in its course： in every instance in which it is exerted an impression made upon the extremities of certain nerves is conveyed to the medulla oblongata or the medulla spinalis； and is reflected along the nerves to parts adjacent to； or remote from； that which has received the impression。
〃It is by this reflex character that the function to which I have alluded is to be distinguished from every other。 There are； in the animal economy； four modes of muscular action； of muscular contraction。 The first is that designated VOLUNTARY： volition； originated in the cerebrum and spontaneous in its acts； extends its influence along the spinal marrow and the motor nerves in a DIRECT LINE to the voluntary muscles。 The SECOND is that of RESPIRATION： like volition； the motive influence in respiration passes in a DIRECT LINE from one point of the nervous system to certain muscles； but as voluntary motion seems to originate in the cerebrum； so the respiratory motions originate in the medulla oblongata： like the voluntary motions； the motions of respirations are spontaneous； they continue； at least； after the eighth pair of nerves have been divided。 The THIRD kind of muscular action in the animal economy is that termed involuntary： it depends upon the principle of irritability and requires the IMMEDIATE application of a stimulus to the nervo…muscular fibre itself。 These three kinds of muscular motion are well known to physiologists； and I believe they are all which have been hitherto pointed out。 There is； however； a FOURTH； which subsists； in part； after the voluntary and respiratory motions have ceased； by the removal of the cerebrum and medulla oblongata； and which is attached to the medulla spinalis； ceasing itself when this is removed； and leaving the irritability undiminished。 In this kind of muscular motion the motive influence does not originate in any central part of the nervous system； but from a distance from that centre； it is neither spontaneous in its action nor direct in its course； it is； on the contrary； EXCITED by the application of appropriate stimuli； which are not； however； applied immediately to the muscular or nervo…muscular fibre； but to certain membraneous parts； whence the impression is carried through the medulla； REFLECTED and reconducted to the part impressed； or conducted to a part remote from it in which muscular contraction is effected。
〃The first three modes of muscular action are known only by actual movements of muscular contractions。 But the reflex function exists as a continuous muscular action； as a power presiding over organs not actually in a state of motion； preserving in some； as the glottis； an open； in others； as the sphincters； a closed form； and in the limbs a due degree of equilibrium or balanced muscular actiona function not； I think； hitherto recognized by physiologists。
The three kinds of muscular motion hitherto known may be distinguished in another way。 The muscles of voluntary motion and of respiration may be excited by stimulating the nerves which supply them； in any part of their course； whether at their source as a part of the medulla oblongata or the medulla spinalis or exterior to the spinal canal： the muscles of involuntary motion are chiefly excited by the actual contact of stimuli。 In the case of the reflex function alone the muscles are excited by a stimulus acting mediately and indirectly in a curved and reflex course； along superficial subcutaneous or submucous nerves proceeding from the medulla。 The first three of these causes of muscular motion may act on detached limbs or muscles。 The last requires the connection with the medulla to be preserved entire。
〃All the kinds of muscular motion may be unduly excited； but the reflex function is peculiar in being excitable in two modes of action； not previously subsisting in the animal economy； as in the case of sneezing； coughing； vomiting； etc。 The reflex function also admits of being permanently diminished or augmented and of taking on some other morbid forms； of which I shall treat hereafter。
〃Before I proceed to the details of the experiments upon which this disposition rests； it may be well to point out several instances in illustration of the various sources of and the modes of muscular action which have been enumerated。 None can be more familiar than the act of swallowing。 Yet how complicated is the act！ The apprehension of the food by the teeth and tongue； etc。； is voluntary； and cannot； therefore； take place in an animal from which the cerebrum is removed。 The transition of food over the glottis and along the middle and lower part of the pharynx depends upon the reflex action： it can take place in animals from which the cerebrum has been removed or the ninth pair of nerves divided； but it requires the connection with the medulla oblongata to be preserved entirely； and the actual contact of some substance which may act as a stimulus： it is attended by the accurate closure of the glottis and by the contraction of the pharynx。 The completion of the act of deglutition is dependent upon the stimulus immediately impressed upon the muscular fibre of the oesophagus； and is the result of excited irritability。
〃However plain these observations may have made the fact that there is a function of the nervous muscular system distinct from sensation； from the voluntary and respiratory motions； and from irritability； it is right； in every such inquiry as the present； that the statements and reasonings should be made with the experiment； as it were； actually before us。 It has already been remarked that the voluntary and respiratory motions are spontaneous； not necessarily requiring the agency of a stimulus。 If； then； an animal can be placed in such circumstances that such motions will certainly not take place； the power of moving remaining； it may be concluded that volition and the motive influence of respiration are annihilated。 Now this is effected by removing the cerebrum and the medulla oblongata。 These facts are fully proved by the experiments of Legallois and M。 Flourens； and by several which I proceed to detail； for the sake of the opportunity afforded by doing so of stating the arguments most clearly。
〃I divided the spinal marrow of a very lively snake between the second and third vertebrae。 The movements of the animal were immediately before extremely vigorous and unintermitted。 From the moment of the division of the spinal marrow it lay perfectly tranquil and motionless； with the exception of occasional gaspings and slight movements of the head。 It became quite evident that this state of quiescence would continue indefinitely were the animal secured from all external impressions。
〃Being now stimulated； the body began to move with great activity； and continued to do so for a considerable time； each change of position or situation bringing some fresh part of the surface of the animal into contact with
