RESPIRATION 485 equally well absorbed if all the nitrogen were removed from the space and the oxygen alone left to fill it. These remarks are equally true of a liquid separated from a gas by moist membranes. "In short, the blood of the pulmonary vessels, regarded as a simple liquid capable of absorbing O and CO,, and apart from the remarkable chemical bodies contained in it, obeys Dalton's law like any other liquid. The blood is, however, not a simple fluid, but contains substances having a peculiar affinity for certain of the gases of the atmosphere ; especially does it contain heemo- globin. It has been stated that the " dissociable " or " respiratory " oxygen of oxy haemoglobin is all yielded to a vacuum at the temperature of the body. This yield differs from the yield of a gas simply dissolved in a liquid (which also is perfect in a vacuum) in not proceeding part passu as the pressure approaches nil. On the contrary in the case of solutions of haemoglobin, as the oxygen pres- sure is diminished at the surface of the solution, no changes occur in the oxyhaemoglobin until the oxygen pressure reaches 25 mm. (about 1 inch) of mercury ; then the oxy haemoglobin yields up its respiratory oxygen. If the steps are retraced, and oxygen is gradually read- mitted to exercise pressure upon the haemoglobin solution, the latter takes up oxygen once more as soon as the oxygen pressure reaches or exceeds 25 mm. of mercury. We may now consider the actual physical conditions of the blood in the lungs. Venous blood is hurried into the capillaries surrounding the air cells ; much of its haemo- globin has been " reduced " (or deprived of its dissociable oxygen), and further it is rich in carbon dioxide, which it has obtained from the active tissues in distant organs. In the alveolar walls it comes into relationship with the air of the alveoli ; probably these are filled with air which never contains less than 10 per cent, of oxygen, and which (in the dog, be it remembered) contains 3*8 per cent, of CO 2 at a time when the expired air contains 2 -8 per cent. Are these conditions such that, owing to the physical laws described, oxygen must pass into the blood, and carbon dioxide out of it 1 ? First, as regards the oxygen. An atmosphere containing 10 per cent, of oxygen implies a partial O-pressure of 76 mm. of mercury (10 per cent, of 760 mm.) ; as this is far above the dissocia- tion point (25 mm.) of oxyhaemoglobin, it is clear that any reduced haemoglobin present would greedily absorb oxygen from such an atmosphere. When the air breathed is much rarefied, the case is different ; the partial O-pres- sure in the alveoli may be so far reduced that the absorp- tion of O by the blood becomes most difficult or impos- sible. As regards the carbon dioxide the matter is not so clear ; but, inasmuch as air drawn from the depth of the lung by means of a catheter contains (in a dog) 3'8 per cent, of C0. 2 , while at the same time the venous blood of the right side of the heart possesses a CO 2 tension of approximately the same percentage, we may assume that C0 2 escapes from the pulmonary capillaries into the alveoli until equilibrium ensues. It is, however, conceivable that the epithelium of the air cells may assist the elimination of CO 2 from the blood by a process of true excretion independently of the above merely physical considerations. So-called Internal Respiration. Venous blood yields up its carbon dioxide and takes up oxygen in the capillaries of the lungs ; arterial blood passes to the general capillaries of the tissues and there yields up its oxygen and receives carbon dioxide. To this act, which in its issues is complementary to the inter- changes of pulmonary respiration, the term internal respira- tion, or respiration of the tissues, has been applied. With as much propriety we might speak of internal urination in reference to the nitrogenous effete matters which the tissues cast into the blood. We are in fact not in a position to form any clear picture of the interchanges which occur between the blood and the tissues other than that which is sketched under NUTRITION (vol. xvii. pp. 678-682). Muscle is the tissue whose metabolic processes are most clearly understood. Living muscle yields no oxygen to a vacuum, although it gives up carbon dioxide freely ; there- fore it is presumably itself in a condition to take up oxy- gen from arterial blood and to give up carbon dioxide to it. But the absorption of oxygen is not immediately necessary to the escape, or the formation, of the carbon dioxide, how- ever much it is so in the last instance. In other words, the oxygen passes into the tissue and is at once combined into some intermediate compound which only at a later stage decomposes and yields carbon dioxide. Whenever an organ is active its blood-vessels dilate and permit a more copious flow of blood through it than when it is at rest. In the salivary glands the blood may even pulsate in the veins and look like true arterial blood in colour. After passing through active muscles the blood contains both more CO 2 and less O than after passing through resting muscle. Dyspnoea and Asphyxia. When the entrance of air to the lungs is entirely pre- vented the phenomena of dyspnoea and asphyxia begin to appear. At first respiration is deeper and more frequent than usual (dyspncea), the extraordinary muscles being called into play in both inspiration and expiration ; the heart beats more quickly at first, but afterwards more slowly ; this is the first stage. It is succeeded by the second stage, in which the violence of respiration is less marked, although the coordination of the act is more irregular ; indeed towards the end of the stage respiratory movements merge in general convulsions of the whole body. Throughout this stage expiration is more marked than inspiration, and the pressure of the blood in the blood- vessels is very great. The third stage is one of exhaustion, which supervenes suddenly, and is marked by loss of con- sciousness, dilated pupils, and absence of the powers of reflex action. The animal seems dead, except that at long intervals feeble inspiratory gasps occur. Finally there comes one great inspiratory effort: the mouth is fixed wide open, the head thrown back, the body arched back- wards, the nostrils dilated, and the pulse after a second or two is indistinguishable (asphyxia). The whole series of events lasts from three to five minutes if the interruption to the entrance of air has been absolute. After death the right side of the heart, with the vessels immediately open- ing into it, viz., the venae cavae and veins of the neck and the pulmonary artery, are engorged with black blood con- taining little or no oxyhaemoglobin. The left side of the heart and the systemic arteries are contracted and empty. All these phenomena are best explained by the known power of venous blood to stimulate the nervous centres. As the blood becomes more and more venous it stimulates more powerfully the great nerve-centres of the medulla oblongata. The respiratory centre is stimulated, especially its expiratory portion ; and, finally, the whole muscular centres of the spinal system are excited, causing general convulsions. The vaso-motor centre is stimulated, causing the rise of blood pressure in the early stage. The slowing of the heart during the close of the first and second stages is due to stimulation of the vagus cardio-inhibitory centre in the medulla. Finally the centres become exhausted from the impurity of the blood bathing them, and their activity fails altogether. Stimulation of the Respiratory Centre. It remains to ask what property of venous blood confers upon it the power of stimulating nerve centres. Venous
