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and general, its influence on the heart was found much greater than that of any chemical agent which was tried. We have seen that suddenly crushing any considerable portion of the brain or spinal marrow instantly destroys the function of that organ.
The conclusions, then, at which we arrive are, that the heart is influenced by all agents applied to any considerable part of the brain or spinal marrow, while the muscles of voluntary motion are only inAuenced by the more powerful agents applied to certain small parts of them.
These facts being ascertained, the other differences observed in the effects of agents applied to the brain and spinal marrow on the heart and muscles of voluntary motion are easily explained.
Irregular action of a muscle arises from stimuli acting pårtially or at intervals on its nerves, or on the part of the brain or spinal marrow from which its nerves arise; but very partial action of an agent on these organs, we have just seen, is incapable of exciting the heart; and while the agent is applied to any part of them, as all their parts seem equally to influence the heart, it cannot act upon it interruptedly, as an instrument does on the muscles of voluntary motioh, when it is moved from place to place in the brain.*
The heart feels the effect of the agent within certain limits, as long as it is applied to the brain and spinal marrow, while the muscles of voluntary motion chiefly feel its first impression ; because they feel only the effects of powerful agents, applied to certain small parts of these organs, which, being strongly impressed, soon lose their excitability; while the heart feels the sum of all, even slight impressions, made on every part of them.
It also appears, from what has been said, why those who have endeavoured to influence the heart by stimulating the small parts of the brain from which its nerves seem chiefly to originate, have failed ; and why the heart may be influenced through this organ and the spinal marrow, after their power is too far reduced to excite the muscles of voluntary motion. As these only obey agents applied to one part, if the change there be not sufficiently great to produce the effect, it can be assisted by no other. Thus I have found by experiment, that a blow which affects the brain generally, without materially injuring it, produces comparatively little effect on the muscles of voluntary motion; but it produces a great effect on the heart, because it feels the sum of all the impressions. The nervous system, therefore, may be so far exhausted as not to admit of the vivid impressions necessary to excite the muscles of voluntary motion, and yet capable of those which influence the heart.
The heart, however, is not the only organ which receives nervous influence from every part of the brain and spinal marrow. The power of the blood vessels, we have seen, may be destroyed by the sudden
It is true, that although the heart is only influenced by agents applied to a large portion of the brain, we may conceive them so applied as to produce irregular action in it, and we find that certain irritations of the nervous system have this effect. Suddenly crushing part of the brain or spinal marrow renders the action of the heart irregular. But it is evident that the heart, not being subject to agents whose action is confined to a small portion of these organs, and being equally affected through all parts of them, must render it much less subject to irregular action, and readily accounts for this not having been observed in the experiments just referred to.
destruction of either of these organs, and it also appears, from direct experiments, that they may be influenced, even in the extremities, by agents applied even to the upper surface of the brain. The alimentary canal may also be influenced through both the brain and spinal marrow. From the extreme irregularity of the motions of this canal, we cannot ascertain whether it is subject to the influence of the different parts of the brain and spinal marrow, in the way in which this was done respecting the heart and blood vessels. I therefore endeavoured to ascertain this point by experiments of a different kind; from which it appears, that on withdrawing a great part of the influence of either the brain or spinal marrow, the stomach is affected in a way which I shall soon have occasion to consider more particularly.
We may easily conceive why the muscles of voluntary motion are excited when those parts of the brain or spinal marrow from which they receive their nerves are stimulated; but it seems at first view more difficult to account for the heart and other muscles of involuntary motion being subject to the influence of every part of these organs. We cannot suppose that they receive nerves from every part of them. We know, indeed, that no organ does so. The following seems to be the state of the question. We see some parts influenced by every part of the brain and spinal marrow; others only by small parts of them. In the latter instances, we see directly proceeding from those small parts of the nerves of the part influenced. In the former instance, namely, where the part is influenced by all parts of the brain and spinal marrow, we do not see nerves going directly from all parts of these organs to the part influenced, but we see this part receiving nerves from a chain of ganglions to which nerves from all parts of them are sent. It is, therefore, evident from direct experiments, that the nerves issuing from ganglions convey to the parts, to which they send nerves, the influence of all the nerves which are received by these bodies.
Such then is the relation which the most important organs of involuntary motion bear to the brain and spinal marrow. Their powers are not directly dependant on either, yet they are subjected to the influence of every part of both, communicated through the medium of the ganglions; and when we see the other organs of involuntary motion equally independent of the brain and spinal marrow, and supplied with nerves from ganglions, in the same way with the former, it is impossible not to infer, that they bear the same relation to the nervous system. Thus, it would appear, that the ganglions may be regarded as a secondary centre of nervous influence, receiving supplies from all parts of the brain and spinal marrow, and sending to certain organs the influence of all those parts.
If the nervous influence of the thoracic and abdominal viscera be thus supplied from a common source, why, in affections of the spinal marrow, it may be asked, is the breathing most influenced when the disease is in the dorsal portion of this organ, and the action of the bladder and rectum, when its chief seat is in the lumbar portion ? This necessarily arises from the intercostal muscles deriving their nerves from the dorsal, and the abdominal muscles from the lumbar portion of the spinal marrow. The latter muscles generally excite, or, at least, increase, the action of the bladder and rectum, by pressing them against their contents, and also by this pressure contribute mechani
cally to expel their contents. Thus, in the above cases, in addition to the failure of nervous influence in the viscera, there is a failure of excitement in the muscles of voluntary motion which conspire with these viscera in certain parts of their functions.
We can trace the communication of nerves issuing from the great chain of ganglions, placed it would seem, to facilitate these communications in the centre of the animal system, with all the nerves of the body. Bichat, although his opinions respecting the use of the ganglions are inconsistent with the results of the experiments just referred to, as well as of others to which I shall have occasion to refer, was induced, from their situation and the distribution of their nerves, to regard them as the centres of nervous systems.
On comparing all the facts on the subject, we have reason to believe, that the system of ganglionic nerves is quite as extensive as that of the nerves proceeding directly from the brain and spinal marrow. We every where find blood vessels which, being influenced equally through the brain and spinal marrow, must receive the nervous influence through the ganglions; and, indeed we can trace the ganglionic nerves attached to and supplying the larger vessels. The following case, related by Dr. Parry, in his treatise on the arterial pulse, might alone be regarded as proving the existence of two sets of nerves in the extremities, the one supplying the organs of voluntary, the other those of involuntary, motion, and strikingly illustrates what has been said on this subject. He observes, “ I have seen a total loss of pulse in one arm, with coldness, but complete power of motion in that part while the other arm was warm, and possessed a perfectly good pulse, but had lost all power of voluntary motion.”
Such then is the manner in which the influence of the nervous system is supplied to the muscles of voluntary and involuntary motion. To the former, from certain small portions of the brain and spinal marrow, and through nerves going directly from these small portions to the muscles; to the latter, from every part of the brain and spinal marrow through a chain of ganglions which, on the one hand, communicate with every part of these organs, and, on the other, with all the muscles of involuntary motion; in the former instance, the influence of the nervous system being the only natural stimulus, in the latter, other stimuli exciting the muscle to its usual function, and the influence of the nervous system, being only occasionally bestowed. on it for purposes which we shall soon have occasion to consider.
When the nerves of a muscle of voluntary motion are divided, the supply of the stimulus on which its function depends being cut off, it is rendered paralytic, not because its power is impaired, for it is as sensible to the effects of stimuli as while its nerves were entire, but because the channel of the only stimulus by which the will operates on it is obstructed, and here the effect of the division of these nerves ends. The consequence is very different, when the nerves of the muscles of involuntary motion, the ganglionic, are divided.
If the principal ganglionic nerves, the eighth pair through which the influence of the brain is chiefly supplied to those muscles be divided, the function of the muscles appears to be wholly unaffected by it. The heart and vessels support the circulation as well as before the division of the nerves. For this result we are prepared by what has been said of these muscles. An evident disorder, however, in the secreting
power of some of the vital organs immediately ensues. The stomach no longer secretes a fluid capable of producing the necessary change on the food, while the fluids of the lungs deviate from the healthy state, and accumulate in the bronchiæ and air cells. The structure of the lungs itself, in the space of a few hours, becomes evidently diseased, and the animal dies of dyspnea; failure in the office of the lungs, necessarily proving more suddenly fatal than failure in that of the stomach. It has been questioned, whether the effects on secreting surfaces of dividing the eighth pair of nerves should be ascribed to the interruption of the influence of the brain, or to the injury done to those surfaces by the act of dividing their nerves. That it produces its effects in the former of these ways, appears from the following facts. When secreting surfaces are deprived of their nervous influence by any other means, the effect is the same; this effect is not at all proportioned to the degree of injury done to the nerves, but to the degree in which the nervous influence is withdrawn; and, as soon as the nervous influence is restored, the surface is again capable of its function. These facts seem sufficient to have answered the question, although it had not been determined by some late experiments, in which Mr. Brodie and Mr. Cutler were so good as to assist me, that it is necessary, after the division of the nerves, to displace one of the divided ends, in order wholly to arrest the function of the secreting surface, the influence of the brain still passing in such a quantity, if this be not done, as to bestow on that surface a considerable degree of the secreting power; and that even when the divided ends, if not otherwise displaced, are separated to a distance of a quarter of an inch.
Thus, we find, that the effect of dividing the ganglionic nerves is of a nature wholly different from that of dividing the cerebral, or spinal nerves; while the division of the latter only deprives the animal of the power of exciting the muscles of voluntary motion, that of the former deranges the functions on which its life depends. Even the structure of the lungs, we have just seen, is evidently disordered in a few hours by the division of the eighth pair of nerves in the neck.
As the function of the stomach is destroyed when the influence of the brain through the eighth pair of nerves is cut off, we should at first view infer, that it is from the brain alone that the stomach derives its nervous influence. But although the process of digestion be suspended by the division of these nerves, it does not follow that the stomach may not derive nervous influence from some other source, because the loss of any considerable part of this influence may destroy its function. Besides, its remaining sensibility, indicated by the efforts to vomit, proves that the influence of the nervous system is not wholly withdrawn from it by dividing these nerves.
If, then, this influence be not supplied to the stomach by the eighth pair of nerves alone, but also, as we have reason to believe from the evidence of anatomy, by nerves arising from different parts of the spinal marrow, it is evident that cutting off the influence of any considerable part of this organ, while we leave the eighth pair of nerves entire, must affect the power of the stomach, though probably not so much, because the brain, we have reason to believe, constitutes the most important part of the nervous system. The same observation applies to the lungs. On appealing to the test of experiment, such was found to be the result; the functions both of the stomach and lungs were impaired, by
destroying any considerable portion of the spinal marrow, the lesion of function being proportioned to the extent and importance of the part destroyed.
Another point relating to this part of the subject remains to be ascertained. Do the effects observed in the stomach and lungs, when part of the spinal marrow is destroyed, arise directly from the destruction of that part, that is, from the ceasing of its office, or from the influence of the brain on the spinal marrow being thus limited ? It is evident, that if the former opinion be correct, the division of the spinal marrow in the middle will not produce the same effects as the destruction of the lower half. If the latter, the effects must be the same. The division of the spinal marrow in the middle produced very little deviation from the healthy state, either in the stomach or lungs, compared with that produced by the destruction of the lower half
Thus, it appears, that the function of the spinal marrow also is necessary to the secreting power, and that, as far as it is necessary to this power, it is independent of any influence derived from the brain. As a partial destruction of the spinal marrow impairs the secreting power, a partial abstraction of the influence of the brain has the same effect. It was found that the division of one of the eighth pair of nerves deranges the function of both the stomach and lungs, nearly in the same degree with the destruction of a certain portion of the spinal marrow. The function of every part of the brain and spinal marrow therefore is necessary to the due performance of secretion.
Here a question of great importance in the animal economy arises. As it appears, from the experiments just referred to, that the nervous power is equally essential with the circulation of the blood, for maintaining the functions of secretion and assimilation, what are the parts they severally perform in these functions? It is evident, that the extreme parts of the sanguiferous and nervous systems are connected in a way very different from that in which these systems are connected in other parts. The heart and vessels of circulation, we have seen, can perform their functions after the influence of the nervous system is withdrawn. The function of the secreting vessels immediately ceases on the interruption of this influence. We must suppose, therefore, either that the influence of the nervous system bestows on the extreme vessels the power of separating and recombining the elementary parts of the blood, or that the vessels only convey the fluids to be operated upon by this influence.
Experiments, to which I have already referred, prove that the most minute vessels which can be seen by a powerful microscope in the web of a frog's foot, are independent of the nervous system. The motion of the blood is as rapid, and the circulation in the foot presents precisely the same appearance after as before the slow destruction of the brain and spinal marrow. If the power of the vessels of secretion had been lost by the interruption of the influence of the nervous system, would not this have necessarily occasioned some change in the distribution and motion of the blood in the web? The conclusion from these experiments is strengthened by others. In those in which the secreting power was destroyed either by the division of the eighth pair of nerves or the destruction of part of the spinal marrow, there Von. I. No. 2.-Museum.