“Apnœa” is the condition of arrested respiration. If a man about to dive into the water breathe deeply and rapidly half a dozen times, he abolishes for a while the desire to breathe. One is naturally inclined to explain this as due to a surplus of oxygen taken into the blood, but a moment’s reflection shows that this cannot be the cause. In the first place, as we have already pointed out, the blood which leaves the lungs in tranquil respiration is very nearly saturated with oxygen. It can take up but little more. Again, the deep inspirations do not change the air in the air-chambers; time is required for the renewal by diffusion of their gaseous contents. It is improbable that the constitution of the air in the alveoli is sensibly altered by a few deep breaths. Probably the explanation is to be found in the effect upon the nerve-centre of distention of the chest. Stretching the nerve-endings of the vagus in the lungs inhibits inspiration. If the stimulation be excessive, inspiration is inhibited for a considerable time. That this is the right theory of apnœa is proved by repeatedly inflating the lungs of an anæsthetized animal with a pair of bellows. The same arrest of inspiration is induced whether the lungs are inflated with air or with a neutral gas, such as nitrogen, so long as the vagus nerve is intact. If this be cut, inflation with a neutral gas no longer produces apnœa. “Dyspnœa” is the term applied to the complex conditions and movements which result from deficient aeration of the blood, or, rather, from the distribution of insufficiently aerated blood to the centres in the medulla oblongata. The blood of the rest of the body may be in a satisfactory condition, but if, owing to ligature of the carotid and vertebral arteries or other causes, the blood supplied to the brain be inadequate to its proper nutrition, the phenomena of dyspnœa are as marked as they are when air is prevented from entering the lungs. That the excitability of the nerve-centres in the brain is greatly increased when this organ is supplied with venous blood, and that their tendency to transmit impulses which call for respiration is consequently exaggerated, is remarkably shown by the following experiment: Two rabbits—A. and B.—are placed under the influence of chloroform. Their carotid arteries are cut, and a crossed circulation established by connecting the proximal ends of A.’s arteries with the distal ends of B.’s, and _vice versa_. The head of each rabbit is now supplied with blood from the heart of the other, the rest of its body by blood from its own heart. A.’s chest is now opened, so that its lungs collapse and cease to take part in respiration. The animal continues to make the movements of respiration in a tranquil manner, whereas B. is thrown into violent dyspnœa. The animal whose brain is receiving aerated blood remains normal, notwithstanding the fact that its lungs and the rest of its body are poisoned with venous blood. The animal whose brain is