Note 91 — Chapter VII · OF THE INFLUENCE OF THE DEATH OF THE LUNGS OVER THAT OF THE BRAIN.

In a preceding article, Bichat maintains that the entrance of the arterial blood contributes to support the action of the brain, principally by the jar which it communicates to this organ. It is astonishing, after this, that he should attribute the suspension of the cerebral functions to the interruption of the chemical phenomena of respiration rather than to that of the mechanical phenomena. He could not however be ignorant, that it is to the last that must be referred the greatest of the two motions with which the brain is constantly agitated.

These motions of the brain in relation with those of respiration have been for a long time observed. Schitling has described them in a memoir inserted in the first volume of the Memoirs of Learned Foreigners. He has shown that the brain rises in expiration, and flattens in inspiration. Haller, Lamure and Lorry have since him investigated this motion, and they have given an explanation of it, which is defective only because they have been ignorant of the influence of respiration on the acceleration of the course of the blood in the arteries through the medium of the capillary vessels.

At the time of a strong expiration, all the pectoral and abdominal organs are compressed, and the arterial blood is forced more especially into the branches of the ascending aorta. This blood goes then in greater abundance towards the head, and has a tendency to pass more quickly in the veins which carry it towards the heart; which would take place immediately if the veins were free. But the pressure made on the pectoral organs has also made the venous blood flow back in the vessels which contain it. Now, this blood has just met that which comes from the arteries; the vessel is distended, and the course of the fluid is arrested in the veins; from that the brain swells and rises up; but as soon as expiration has ceased, the dilatation which takes place in the chest attracts, in some measure, the blood of the superior venæ cavæ; the veins which enter it are soon emptied and the brain flattens down.

In reflecting on the mechanism by which this movement of the brain is effected by the influence of respiration, we cannot perceive why the phenomenon should be limited to the organ contained within the cranium, and especially why the spinal marrow should not equally partake of it. The continuity of this organ with the cerebrum and cerebellum, its situation in a cavity which it does not entirely fill, the numerous arteries which it receives from the intercostal and vertebral arteries, the number and size of its veins destitute of valves are so many circumstances which should favour the accumulation of the blood at the time of expiration, and consequently produce its swelling. For the purpose of seeing if my conjectures were well founded, I have made some experiments; I laid bare in a young rabbit the spinal marrow at about the eighth or ninth dorsal vertebra, I saw it perfectly whole and surrounded by its coverings. At first I perceived no motion, but soon the animal being much incommoded by the position in which I kept him, made a deep inspiration, and then I saw distinctly the spinal marrow flatten, and a small vacuum between the dura mater and the osseous parietes of the vertebral canal. In the following expiration, the spinal marrow resumed its original size. I was unable to see any thing more in this animal.

I laid bare in a dog of middle size, the spinal marrow, a little above the lumbar region; I could not mistake there a very evident motion, in relation with respiration: a flattening during inspiration, and a swelling during expiration. The phenomenon was so marked, that the air entered the vertebral canal with a noise, whilst the animal inspired, and was forced out when the animal expelled the air from his lungs.

For the purpose of satisfying myself that this motion took place in the spinal marrow and not in the dura-mater, I cut this membrane in the whole extent of the opening made in the vertebral canal, and I was able easily to convince myself that the motion was from the swelling of the spinal marrow. I am not however certain that there is not a slight rising of the organ from the dilatation of the large veins in the anterior part of the vertebral canal, but this dilatation cannot be considerable, on account of the fibrous layer which covers the posterior face of these veins.