Cardiac branches.—We have in a former part of this work (article EA KT) had occasion to state that several celebrated physiologists have failed in exciting the muscular contmctions of the heart by irritation of the trunk of the vagus before it gives off its cardiac branches, or of these cardiac branches themselves. We have very frequently repeated this experiment upon animals immediately after death, and we havenot been able to satisfy ourselves that galvanic and mechanical excitation of these nerves has any effect in renewingor increasing the contractions of the heart. No doubt we have not unfrequently seen the contractions of the heart become more frequent and vigorous during the performance of this experiment ; but as similar changes in the strength and rapidity of its contractions are occasionally observed in an animal after death when no artificial excitant has been applied to these nerves, and from causes which cannot at present be explained, we did not think ourselves entitled to attribute these changes in the heart's action to die excitation of the nerves. Valen tin* has stated that he has produced muscular contractions in the heart in different animals .by irritation of the trunk of the vagus. lie also statest that similar contractions of the heart were produced by excitation of the spinal ac cessory and of the three superior (sometimes also of the fourth) cervical nerves, and he main tains that the motor portion of the cardiac nerves comes from the spinal accessory and the superior cervical nerves. Longett mentions that he failed in influencing the rhythm of the heart by the application of galvanism to the vagi in dogs, rabbits, and sheep, but very fre quently succeeded by scraping the cervical cardiac branches of the vagus. Allowing that it is possible to increase the contmctions of the heart by galvanic or mechanical excitation of the vag,us or its cardiac branches, it must be admitted by every one that there is a veiy marked difference between the heart and volun tary muscles in this respect, for all those who have failed in their experiments on the nerves of the heart, have felt not the smallest difficulty in producing contractions of the voluntary muscles by excitation of their nerves. The in creased frequency of the pulsations of the heart observed during and for some minutes after the division of the vagi may be fairly referred to the struggles and terror of the animal, and the feeble and rapid pulsation of the heart which precedes death from this experiment is not owing to any direct effect upon that organ. The sudden death occasionally remarked after the division of these nerves, and which some of the early experimenters attributed to arrest ment of the contractility of the heart, was in fact dependent upon the suffocation of the animal by the suspension of the movements of the muscles which dilate the superior aperture of the larynx. We have related several expe riments which appear to prove that when in juries of the brain and mental emotions affect the contractility of the heart, the nervous influ ence is not transinitted by the cardiac branches of the vagi alone, but may also pass along the filaments of the sympathetic or ganglionic system of nerves.
Pulmonary branehes.—Do the pulmonary branches of the vagus contain motor filaments ? We have made various unsuccessful attempts to produce contractions in the muscular fibres of the bronchial tubes by excitation of the vagi in the neck.* Dr. C. T. B. Williamst was also unsuccessful on attempting this experiment, though he succeeded in producing contractions in the bronchial muscular fibres by their direct excitation, as by transmitting galvanism through the substance of the lungs, Ike. Longet and Volkmann have not only succeeded in exciting contmctions of the muscular fibres of the bronchii by direct stimulation, but also by ex citants applied to the branches of the vagus.r To what extent are the pulmonary branches of the vagus sensiferous ? Bmchet relates some experiments which seem to prove that the sen sation arising from the want of fresh air in the lungs, or the besoin de respirer, is annihilated by the division of vagi.§ Mr. Graingell re peated one of Bmchet's experiments, and seemed satisfied that his conclusions were correct. There are certain sources of fallacy attending the mode in which Bmchet performed his ex periments against which he has not taken the necessary precautions.
We have satisfied ourselves by numerous experiments that the sense of anxiety arising, from the want of fresh air in the lungs conti nues after dividing the vagi when the access of air to the lungs is prevented ;111. and Volkmann" and Longettt from their experiments have also arrived at the same conclusion. It is possible that certain impressions which may excite the besoin de respirer are conveyed upwards to the encephalon through the medium of the sympa thetic, but it is more probable that in the con ditions induced by the experiment it was more immediately dependent upon the circulation of ill-arterialized blood through the tissues of the body, and more especially through the ence phalon. We do not mean to deny that im pressions conveyed along the vagi to the ence phalon may not excite the besoin de respircr ; on the other hand, we believe that it is very prob able that this sensation as first felt, when the respiration is suspended for a short time in the healthy condition of the body, is dependent upon impressions conveyed along this nerve. When, however, the respiration has been suspended for a longer time and venous blood begins to circulate along the arteries, the other excitants of the besoin de respirer come into operation.
Brachet,* Krimer,t and Longet4 have from their 'experiments arrived at the conclusion that the sensations occasioned by irritation of the inner surface of the trachea and bronchial tubes, and which usually precede coughing, are anni hilated by dividing the vagi. We have made repeated experiments on this point, and though we could not satisfy ourselves that these sensa tions were affected to the extent maintained by these authors, we believe that they are at least blunted.
To what extent do the filaments of the vagi act as incident nerves?—lt has been proved by the expeiiments 9f Legallois,§ Flourens,Il and others, that all the respiratory and muscular movements cease on destroying the medulla oblongata, though the other parts of the ence phalon situated above this may be injured in various ways without necessarily producing this effect. It is further well known that if the spinal chord be cut across, all the respiratory muscles are paralysed which receive their nerves from that portion of it below the point where it was divided, while those muscles vvhich receive their nerves from that portion of the spinal chord still continuous with the me dulla oblongata perform their usual functions. From these and other facts it may be consi dered as ascertained that all impressions made at the lungs and elsewhere capable of causing respiratory movements, must be conveyed to the medulla oblongata before they can produce any reflex excitation of the muscles of respira tion. That the vagi can convey these impres sions from the lungs is not only rendered pro bable from their attachment to the medulla oblongata, but may almost be considered as proved by the result of the experiments upon the spinal chord to which we have just referred. It, however, by no means follows that the vagi are the sole excitant nerves of respiration. It has been fully ascertained by numerous expe rimenters, more especially by those who have investigated the functions of this nerve from the time of Legallois, that an animal will continue to breathe after the division of both vagi in the neck, if care be taken Up secure the ingress and egress of air to and from the lungs. It is now well known, as we have already had occasion to point out in examining the functions of the laryngeal branches, that if the vagi be injured above the origin of the recurrent laryngeals, none of the muscles attached to the arytenoid cartilages can any longer act in unison with the muscles of respiration, all their movements cease, and the superior aperture of the ,larynx can no longer be dilated during inspiration. If the larynx be large, and the animal refrain from any violent effort, an adequate quantity of air may still find its way to the lungs, and the respirations are at first performed with ease. If, on the other hand, the larynx be small, its superior aperture may be 'mechanically closed and the animal may be immediately suffocated, or the air may still pass through the larynx but in diminished quantity, and the animal may labour under dypsncea from the moment the nerves are divided, up to its death. Even when means are taken to secure the free entrance of air into the lungs, an immediate and marked diminution in the frequency of the respiratory movements follows the division of both vagi in the neck. A. G. F. Emmert concluded, but apparently more upon theoretical grounds than from any direct observations made in the two experiments he had at that time performed on rabbits, that after lesion of the vagi the respira tions become less frequent and prolonged.* Mayer reckoned the number of respirations, both before and at various periods after section of these nerves in five experiments upon the ass, dog, and rabbit, and found a very marked diminution in their frequency after dividing the nerves.t Mr. Broughton mentions, that in a horse in which the vagi were divided " the respirations became slow, twelve in a minute ;" and in another horse " the respirations fell to five in the minute.% At what period after the division of the nerves these last observations were made, and what was the number of the respirations previous to the commencement of the experiments, we are not informed. Sir Astley Cooper has given the result of two expe riments upon rabbits which well illustrate the effect of the division of the vagi upon the respi ratory movements.§ In our experiments we ascertained that the diminution in the frequency of the respiratory movements, generally to less than half of their former number, is an imme diate effect of the di vision of both vagi. The respi ratory movements seem to be performed more slowly, and, generally, even from the first, in a somewhat heaving manner.11 Arnold in his expe riments upon hens also observed a very consi derable diminution in the frequency of the re spiratory movements.5 Brachet has asserted** that an animal continues to breathe after sec tion of the vagi, because it has acquired the habit of using the respiratory muscles. Dr. -Marshall Hall has maintained that after the vagi are divided the respiratory movements are no longer a function of the excito-motory but of the cerebral portion of the nervous system and he has adduced in support of this view the statement of Cruveilliier that after the function of volition has been suspended by destroying the cerebrum, the respiratory movements are instantly arrested on dividing the vagi near their origin.* In putting this opinion to the test of experiment we found that though the respirations were very much diminished in frequency by the removal of the cerebrum and cerebellum and then dividing the vagi, they nevenheless continued for a longer or shorter timed- Similar results have also been subsequently obtained by Volk mann,j by Flourens,§ and by Longetil From these facts we are entitled to conclude that the vagi are not the sole excitors of respiration, and that impressions may be made upon the medulla oblongata capable of exciting the involuntary respiratory movements after the vagi have been divided in the neck, and when impressions made on their expanded extremities in the lungs can no longer be conveyed inwards to the central organs of the nervous system. The importance of the vagi as incident nerves of respiration is not only proved by the marked and immediate diminution in the number of the respirations which follows their division,11" but also in a more striking manner by the morbid changes which take place in the lungs.