Boerhaave thought, that death which follows the injection of air into the veins was always owing, as it is in this case, to the presence of the air, which offers, in the small vessels, a mechanical obstacle to the passage of the venous blood. CHAPTER III. OF THE INFLUENCE OF THE DEATH OF THE HEART OVER THAT OF THE LUNGS. The lungs are the seat of two very different sorts of phenomena. The first, which are entirely mechanical, are relative to the rise and fall of the ribs and diaphragm, to the dilatation and contraction of the air vessels, and to the entry and exit of the air, which is the effect of these movements. The second, which are purely chemical, may be referred to the different alterations, which the air and blood experience. These two sorts of phenomena have a mutual dependence on each other. Without the mechanical, the chemical changes could not be made; without the chemical changes, the blood would cease to become an excitant to the brain, in consequence of which that organ would no longer operate upon the diaphragm or intercostal muscles; the muscles themselves would then become inactive, and the motions of the thorax be annihilated. These phenomena, however, are put an end to in a different manner by the death of the heart, accordingly as it happens on one or the other side. I. _In what manner are the actions of the lungs interrupted, when the black-blooded heart ceases to act?_ The heart has certainly no influence over the mechanical functions of the lungs, but it contributes essentially to produce the chemical changes which are made there, by sending thither the fluid which is destined to undergo a change. When its functions then are interrupted as may happen from wounds or be occasioned by ligature, the chemical changes which should be made in the blood, are suddenly suppressed; though the air continue to enter into the lungs, from the dilatation and contraction of the chest. Meanwhile there arrives nothing at the red-blooded heart, or[68] so little as to be insufficient for the production of the cerebral movements. The functions of the brain are consequently suspended, and of course the movements of the diaphragm and ribs. II. _In what manner are the actions of the lungs interrupted, when those of the red-blooded heart are suspended?_ Whenever from wound, ligature, or aneurism, the functions of the red-blooded heart or aorta cease, the functions of the lungs are terminated in the following order: 1st. There is no further impulse made upon the brain. 2dly, No further movement of that organ.[69] 3dly, No further action exercised upon the muscles. 4thly, No further contraction of the intercostals or diaphragm. 5thly, The mechanical functions of the lungs cease. 6thly, Their chemical functions cease.