The state of our knowledge does not, it must be confessed, permit us to offer a satisfactory explanation of the cause of the above-men tioned phenomena. We have already stated reasons against regarding the stagnation of the blood in the lungs in asphyxia as attributable to a loss of the supposed vital power of motion belonging to the blood in the capillary vessels: and we think it quite as just to regard the stag nation as the effect of over-stimulation and constriction of the minute vessels of the lungs by the dark blood, as to attribute it, in the manner some have done, to the deficiency of that stimulation which arterial blood, without any good reason, is presumed by them to give to the small vessels.
2. Circulation within the cranium. — The limits of this essay do not permit us to do more than allude very shortly to the nature of the circulation within the cranium,—a subject, in some respects, nearly related to the facts just stated, and of great importance from the general dependence of the state of the cerebral func tions upon the quantity and force of blood which flows through the brain.
The bloodvessels within the cranium are dif ferently situated from those in other parts of the body in this respect, that they are removed from the influence of atmospheric pressure. In consequence of the unyielding nature of the skull, and its being closed on all sides, except ing at the places where the nerves and blood vessels pass through the bones, the cavity of the skull must necessarily be equally full at all times ; and the spinal canal is in the same pre dicament.
The whole quantity of fluid or solid matter, then, within the cavity of the cranium and spinal canal must be always the same; or, during the circulation just as much blood must issue as enters it, and it is physically impossible to increase or diminish the whole quantity con tained in the brain by increased pressure, by opening. of an artery or vein or any other means. It was shewn by various well devised experi ments performed by the late Dr. Kellie,* that in animals bled to death, while the rest of the body was exsangueous, the brain retained its usual appearance so long as the vault of the cranium was entire, but that a perforation of the skull, such as to allow the atmospheric pressure to act upon the brain and bloodvessels of the head, caused the evacuation of blood from the head as from other parts of the body.
While the whole bulk of the contents of the cranium, however, must necessarily remain the same, yet the relative quantity of arterial and venous blood may vary within a short space of time, the pressure exerted by the blood in the vessels may be greater or less accordino- to cir cumstances ; and there may occur within the skull local determinations or partial distribu tions of the blood. When from rupture of a
bloodvessel, inflammation, suppuration, or other causes, blood, serum, or pus are effused into the cavity of the cranium, the circulating blood must be diminished in quantity ; when there is any obstruction to the return of the blood by the jugular veins, the pressure of the blood en tering by the carotid artery is proportionally greater; and when the arteries which supply blood to the brain are obstructed, or the heart's action is less forcible than usual, the pressure on the brain must be. diminished in a corre sponding degree.
In the natural state of the circulation the pressure exerted by the blood circulating through the cranium is subject to regular alter nations of increase and decrease from the effect of the heart's action and the motions of respira tion. When the brain of man or of animals is exposed by the removal of a part of the skull, it is seen to be slightly raised at the exposed part at each arterial pulsation, and more perceptibly during each expiration. The brain falls again during each succeeding inspiration, but does not sink below the level of the skull. These motions may also be perceived at the fontanelles of the infant's head, where the bony parietes of the skull are deficient. In the closed state of the skull, for the reasons previously mentioned, it is obvious that there can be no motions simi lar to those observed in the brain when ex posed, but nevertheless the brain must be more forcibly pressed upon by the blood at these times than at others. Haller, who had observed these motions, conceived the depression during inspiration to be caused simply by the ease with which the blood enters the chest at that time, and attributed the swellino. of the brain during expiration to the obstacle then offered to the descent of the blood through the jugu lar veins. It seems, however, probable that the greater fulness of the arteries during expiration may also contribute to raise the brain at the time when the collapse of the walls of the chest occurs : for Magendie ob served, that when a ligature was put upon the jugular vein, the blood which issued from this vein by an aperture above the ligature, flowed with greater force during expiration, shewing that increased arterial pressure during expira tion was continued through the capillaries into the veins. Sign. Ravina, who made a very extensive series of experiments upon these mo tions, found that when the brain has been de pressed during inspiration, it again swells, although no expiration succeeds, but that when raised during expiration, it does not again sink, if inspiration does not follow.