[78] The syncope is produced in this case, from the sudden change in the circulation of the brain. But this change varies according to the seat of the effusion. If it be in the peritoneal cavity, the pressure that it makes interrupts the circulation in all the organs contained in the abdomen; the descending aorta is found compressed, and the blood, forced back towards the superior parts, accumulates in the sinuses and vessels of the brain. If the fluid be evacuated by puncture, the equilibrium is re-established in the different parts of the vascular system, the blood enters vessels which were before closed to it, it abandons in part those of the brain, and it is this sudden change in the circulation of the organ which produces syncope. If, on the contrary, the effusion be formed between the two layers of the arachnoides, and we can, as in spina bifida, evacuate the fluid by puncture, the vessels of the brain are immediately relieved of the pressure to which they had been subjected, and the blood, which before was forced back, towards the inferior parts, is driven forcibly into them; the change is, as must be perceived, the reverse of the preceding; but the result is the same, and syncope is produced in this case as in the other. CHAPTER VI. OF THE INFLUENCE OF THE DEATH OF THE LUNGS OVER THAT OF THE HEART. We have already said, that the functions of the lungs are of two kinds, mechanical and chemical. Now the activity of this organ ceases sometimes by the former, and sometimes by the latter of these functions. Any wound, which exposes the lungs on both sides, for a considerable extent, occasions their sudden collapse; any division of the spinal marrow, which suddenly paralyses the intercostals, and the diaphragm; any very strong compression exerted at the same time upon the whole of the thorax, and the parietes of the abdomen, any sudden injection of a large quantity of fluid into this cavity, are all of them causes which begin the death of the lungs, by putting an end to their mechanical functions. Those which influence in the first place their chemical functions, are the different sorts of asphyxia, strangulation, submersion, and a vacuum, in whatever manner produced. I. _In what manner is the death of the heart occasioned by the interruption of the mechanical functions of the lungs?_ The interruption of the functions of the heart, can only succeed in two ways to that of the mechanical functions of the lungs: 1st. Directly, because a mechanical impediment is put to the circulation of the blood, by a state of collapse in the lungs. 2dly. Indirectly, because in such state the lungs no longer receive the materials, upon which their chemical functions are exerted, and therefore cannot transmit them to the heart.