Chapter II · OF THE INFLUENCE OF THE DEATH OF THE HEART, OVER THAT OF THE BRAIN.
F. Gold translation
CHAPTER II. OF THE INFLUENCE OF THE DEATH OF THE HEART, OVER THAT OF THE BRAIN.
I shall evidently have determined what is the mode of this influence, should I be enabled to establish in what way the action of the heart is necessary to that of the brain; for in this instance the cause of death will be no other than the privation of the cause of life. Now the heart can only act upon the brain in two ways; by the nerves, or the vessels which serve as their connecting medium. In fact these two organs have no other means of communication.
It is evident that the nerves cannot be the agents of such actions; it is the province of the brain to act by means of the nerves. The different parts of the body never influence the brain by such means, excepting in the sympathies. If a bundle of nerves belonging to the voluntary muscles be tied, the muscles indeed will cease to act, but nothing will be changed in the cerebral mass.[56]
I have ascertained by many experiments that the phenomena of galvanism, which are propagated so energetically from the brain towards the organs, which descend, if I may so express myself, along the nerve, will hardly ascend in a contrary direction. Apply the apparatus to a nerve of the loins and the muscles of the upper limbs, and when the communication is made, there will be scarcely any contraction; but on the establishment of a communication between the same nerve and the lower limbs, a violent convulsive motion will instantly be occasioned. I have even observed, on placing two metallic plates, the one under the lumbar nerves, and the other under the upper limbs, that the communication of the two plates by means of a third metal, will cause a contraction of the lower limbs, while the upper limbs remain inactive, or move but feeble.[57]
These experiments are particularly applicable to the relation of the heart with the brain; for not only is it true that the section, ligature or compression of the cardiac nerves are of little effect with regard to the functions of the latter, but it is true also, as we shall presently see, that they do not directly modify the movements of the former. We may conclude that the vessels are the exclusive agents of the influence of the heart upon the brain.
The vessels, as every one knows, are of two sorts—venous or arterial—they carry black or red blood, the latter answer to the left side, the former to the right side of the heart. Now their functions being very different, the action of one of the portions of this organ on the brain, can never be the same as that of the other portion. We shall inquire in what way they both of them act upon it.
In naming these two portions, I shall not make use of the expressions of right and left to distinguish them, but of those of the red-blooded and the black-blooded heart, for each of these portions of itself is an isolated organ, distinct from that to which it is applied, and in the adult especially so. In fact there are two hearts, the one arterial, the other venous, notwithstanding which, we can hardly employ these adjectives for the purpose of designating them, since they both alike possess their arterial and venous appendages. On the other hand, they are neither of them situated exactly to the right or to the left, are neither of them exactly forwards or backwards. Besides which these latter denominations would not apply to animals.
I. In what way does the cessation of the functions of the red-blooded heart interrupt the functions of the brain?
The red-blooded ventricle and auricle, exert their influence upon the brain by means of the fluid which they send thither through the carotid and vertebral arteries. This fluid may excite the cerebral organ in two ways. 1st. By the movement, with which it is directed. 2nd. By the nature of its colouring principle.
It is easy to prove that the movement of the blood is necessary to the life of the brain. Expose the brain of an animal in part, and tie the carotids. In such case the cerebral movement will be sometimes weakened, and then the animal will be stupified, at other times the vertebral arteries will exactly supply the place of the carotids, and then there will be nothing deranged in the principal functions of the brain; for there is always a relation existing between the alternate rise and fall of the cerebral mass, and the energy of life which it displays.[58]
In general, the obliteration of the carotids is never suddenly mortal. Animals will live without them, at least for a certain time. I have kept dogs in this state for several days and have afterwards made use of them for other experiments: two however died in the course of six hours, after the application of the ligatures.
After having made the above experiments which go very far to the establishment of the principle which I am labouring to prove, let a piece of the cranium be taken from another animal and tie the vertebral and carotid arteries. The movement of the brain will then be entirely interrupted and the animal immediately die.
The impulse, which proceeds then from the influx of the blood into the brain, is a condition essential to the functions of this organ, but other proofs may be adduced, for the establishment of the truth of this assertion.
1st. There are a number of compressions, which can only act by preventing the brain from being duly affected by such impulse. A collection of pus, or blood, will often put a stop to all the functions, which relate to the perception, memory, and voluntary motions of the individual. Let such compression be removed and his sensibility will immediately re-appear. In such case, it is manifest that the brain was not disorganized, but only compressed, and in a state incapable of being excited by the heart.[59]
I do not think it necessary on this subject to cite cases. All authors, who have treated of wounds of the head, are full of them. I shall content myself with remarking, that the same effect may be artificially produced in our experiments upon animals, and that accordingly as the brain is compressed or free, the creature will be insensible, or the contrary. According to the degree of the compression, will be the degree of the stupor.
2dly. There are reptiles, in the brain of which no motion whatever is occasioned by the heart. The frog is of this species. On raising the upper portion of the cranium, and exposing the brain, there cannot be perceived the slightest motion. Now in this species, and that of the salamander, the influx of blood may be cut off from the cerebral organ without occasioning the immediate death of the animal. The voluntary muscles for instance continue to act; the eyes to exhibit a lively appearance, the tact also of the creature is manifest for some time after the heart has been taken away, or the double branch which proceeds from the single ventricle of these animals has been tied.[60] I have frequently repeated these experiments, and have constantly found the effect the same.
3rd. It is a general observation, that those animals which have a long neck, and in which the heart for that very reason is not so capable of exerting a lively influence over the brain, have a more limited intellect, and the cerebral functions less marked. On the contrary a very short neck, and the approximation of the heart to the brain very generally are found to coincide with the latter. Similar phenomena are sometimes observed in men. They who have the neck particularly long are dull, they who have it short, for the most part intelligent and lively.
From these many facts we may confidently assert, that one of the means, by which the heart maintains the brain in action, consists in the habitual movement, which it impresses on it.
But this movement is essentially different from that which in the other viscera, such as the liver, or spleen, is derived from the same cause. In these it is little manifest, in the brain it is very apparent; the reason is evident; the large arterial trunks of the brain, are situated at its base, between the brain and its bony parietes; in consequence of which, at each diastole, the vessels experience a resistance from the bone, which is communicated immediately to the cerebral mass. At such time the brain is really lifted, just in the same way as we see a tumour lifted by the arteries which creep along the bones beneath it; and instances of this are frequent. So apparent indeed is the motion of tumours when they are situated over the carotid, as it lies upon the vertebral column, or over the femoral artery, immediately after its passage under the crural arch, as often to occasion doubts with respect to their nature.
But no other organ is enclosed within a bony cavity; the motion of the arteries every where else, is lost in the surrounding cellular substance, or soft parts. Such motion, then, is unessential to the functions of the liver, the kidney, and other analogous viscera.
The integrity of the functions of the brain, is not only dependent on the mere motion, but on the sum also of the motion communicated. It is equally impaired by too much, or by too little motion. Of this assertion the following experiments are proofs.
1st. Inject water by the carotid of a dog; the presence of this fluid in the brain is not pernicious, and the animal will live very well, when the injection has been skilfully made. But if it be pushed with violence, the cerebral action will immediately be troubled, and often cannot be restored.[61] In every experiment, there will be found to exist a relation between the force of the impulse and the state of the brain; if the pressure be but a little augmented, its effects will be instantly seen in the agitation of the countenance of the creature; if relaxed, a corresponding calm will succeed; if increased to the highest pitch, it will immediately occasion death.
2dly. If the brain be exposed, and an artery afterwards opened, so as to produce a considerable hemorrhage, the motion of the brain will be diminished in proportion as the afflux of the blood to it is diminished, and finally will cease entirely. Now, according to all these various degrees of diminution, which may be observed in the movements of the brain, will be the corresponding weakness of the cerebral influence as it is discoverable in the state of the eyes, the touch, and the voluntary motion of the animal.[62]
Hence it is easy to see, why a state of prostration and languor is always the consequence of great hemorrhage—and from what has been said above we may conceive the reason, why the arterial system of the brain has been at first concentrated at its base, while the larger venous trunks are almost all of them situated on the convexity of its surface. The base of the brain is small and easily moved, the convexity large and little capable of transmitting motion, such as could be made upon it by vessels. Besides, it is at the lower part of the brain that exist its particular and essential forms. The lesions of these are mortal, and consequently their functions must be important. On the contrary, experiment and observation alike have proved, that very little derangement follows, from cutting or rending the substance of the upper part of this organ. Hence also we may see the reason, why its natural defences towards its base, are constituted in such way as to be almost impenetrable, and why at its upper surface, it is less protected. Now, where its life is indispensable, and its action absolutely necessary, it should naturally receive the first and undiminished impulse of its excitant. We may conclude, that the interruption of the action of the red-blooded heart is the occasion of interruption in the action of the brain by annihilating its movement.
But this movement is not the only means by which the influence of the heart is exerted on the brain; for if it were so, we might easily reanimate the enfeebled functions of the latter, by injecting it with water at the same time through both the carotids. If pushed with an equal force, the black blood and the red blood alike would be capable of keeping up its action; but this, as we shall presently see, is not the fact.
The heart, then, acts upon the brain by the nature of the fluid which it sends thither; but as the lungs are the focus, where the blood undergoes an alteration, we shall refer the examination of its influence upon the cephalic system, to the chapter in which we shall treat of the relation of this system, with that of the lungs.
II. In what way does the cessation of the functions of the black-blooded heart interrupt the functions of the brain?
It very rarely happens that general death commences by that of the venous auricle and ventricle. On the contrary, they are almost always the last in action, and when they cease to act, the brain, the lungs, and the red-blooded heart have already ceased to exhibit their respective phenomena. Nevertheless the contraction of these cavities may be annihilated, or rendered at least inefficacious with regard to the circulation, from the rupture of an aneurism or similar causes; in which case the brain becomes inactive and dies, as we have shewn it to do in the preceding section, from want of movement.
There is another kind of death of the brain depending on the interruption of the transmission of blood from the head to the heart, as when the jugulars are tied. The venous system, in consequence, is glutted and the brain compressed, from the continued afflux of the red blood into its arteries; but the phenomena of this sort of death are already sufficiently known.
In the present chapter it is my intention to examine a species of death, the principle of which by many physiologists has been placed in the heart, but which appears to me to affect the head only; I mean that death which may be occasioned by the injection of air into the veins.
It is generally known, that as soon as any quantity of this fluid is introduced into the vascular system, the movements of the heart are accelerated, that the creature is much agitated, cries with pain, is convulsed, and soon after deprived of its animal life, but lives organically for a certain time, and then invariably dies.[63] Now, what is the organ so readily affected by the contact of air? I affirm it to be the brain, and not the heart; and maintain that the circulation is annihilated, only because the cerebral actions have previously been so.
For, in the first place, in this kind of death, the heart continues to beat for some time after the cessation of the animal life, and consequently for some time after that of the action of the brain.[64]
Secondly, By injecting air into the brain through one of the carotids, I have caused the death of the creature just in the same way as when air is introduced into the veins; excepting only with a previous palpitation of the heart.[65]
Thirdly, Morgagni has cited a number of cases of sudden death, the cause of which should appear, from his remarks, to be the repletion of the blood vessels of the brain by air, which had been developed there spontaneously, and which he says, by its rarefaction, compressed the origin of the nerves. I cannot suppose that such compression can be effected by the very small quantity of air, which, when injected into the carotid, is sufficient to occasion death; accordingly, I should doubt whether this compression were real in the cases adduced, but for this, they are not the less important. Whatever be the manner in which it kills, air is fatal whenever introduced into the brain, and this is the essential point. It is with the fact that we have to do and not the manner.[66]
Fourthly, As often as an animal is killed by the insufflation of air into one of its veins, I have ascertained that the whole of the red-blooded, as well as the black-blooded heart, is full of a frothy blood, mixed with air bubbles; and that the carotids, and vessels of the head, contain a similar blood; such blood must act upon the brain, in the same manner as it does in the two sorts of apoplexy, of which we have just been making mention.
Fifthly, If air be pushed into one of the divisions of the vena portæ from the side of the liver, it oscillates in the greater trunks of that organ for a considerable length of time, and arrives but slowly at the heart.—In this instance I have observed, that the animal experiences, only after a certain interval, those affections which are sudden when the fluid is injected into the veins of the principal system.[67]
Sixthly, The rapidity with which, in certain experiments, the annihilation of the cerebral action succeeds to the insufflation of air into the veins, might almost persuade us that such phenomenon is occasioned, as it is in wounds of the heart and syncope;—but 1st. The most simple inspection is sufficient to shew us that the heart continues to act after the apparent death of the animal.—2dly. As the motions of the heart are prodigiously accelerated by the contact of the foreign fluid, they push on the frothy blood with an extreme velocity, and hence we have the reason, why the brain in such case is so rapidly affected.
Seventhly, Were the cerebral action in this sort of death interrupted for want of movement from the heart, it would happen as it does in great hemorrhages of the aorta; that is to say, without violent convulsion. But here, on the contrary, the convulsion is extremely violent, immediately after the injection, and consequently, announces the presence of an irritating substance on the brain.
We shall conclude, that in the accidental mixture of air with the blood of the venous system, it is the brain which dies the first, and that the death of the heart is the consequence of the death of the brain. I shall explain in another place, in what way this phenomenon is occasioned.