AFFINITY OF THE BLOOD FOR THE TISSUES. 277 oxygen which is in the air. Having satisfied this, and become arterial, it is pressed on by the advancing column. If the reader will station himself at the door of a theatre, and watch the column of eager playgoers struggling to get to the money-takers, he will see an image of the forces of the circulation. Each visitor is anxious to put down his half-crown in exchange for a ticket. No sooner has he satisfied that "affinity," than he finds himself pressed forward by the man behind him, still in a state of unsatisfied affinity; and so the rush continues. An image is not an explanation, but it may render an hypothesis more intelligible; and having attempted to make Professor Draper's hypothesis intelligible, we will add, by way of criticism, that it has one serious defect. It rests on the notion of chemical affinity, yet chemical affinity acts only at insensible distances, and here the distances are sensible. That liquids should circulate in a tube, when one of them has a greater affinity than the other for the walls of that tube, is not evidence that the liquids will circulate in a tube in virtue of an affinity supposed to exist between one of them and the tissues outside the tube, because these tissues, being at sensible distances, cannot exert their affinity. If instead of " affinity we substitute "leakage "—if we remember that the action of endosmose is necessarily set up between the blood and the tissue-plasma, the hypothesis may be more acceptable. Spallanzani, in his celebrated Mémoires sur la Respiration, relates that, when snails were confined in vessels, and had absorbed all the oxygen from the contained air, the movement of their lungs ceased, and with it ceased all movement of the heart—the circulation was arrested. He had only to remove the top of their shells, which could be effected without injury, and the phenomenon was easily watched. By keeping a snail thus confined, at a temperature gradually diminishing, the gradual diminution of the respiration and circulation became very evident. When the temperature fell to zero, the heart ceased to beat altogether, and the blood was stagnant in the veins. In this state of suspended animation the animal was kept for several hours; but on raising the temperature, the lungs began once more to inflate, the heart to beat, the blood to circulate; and, as in the palace of the Sleeping Beauty, all was vivid activity where a minute before all was the image of death. "The hedge broke in, the banner blew, The same effect of torpor was produced by the absence of oxygen and the absence of heat. Spallanzani placed a snail in a vessel containing mephitic gas. In eleven minutes the heart was still, and remained so during five hours. On reintroducing atmospheric air the lungs once more began to move, and life returned. To prove that it was the oxygen of the air, and nothing else, which caused this reanimation, Spallanzani repeated the experiment, substituting nitrogen gas for atmospheric air, as the replacer of the mephitic gas; but no movement was visible. Thus it appeared that the animal ceased to breathe because it had ceased to absorb oxygen. It ceased to absorb oxygen under two conditions→ when there was none present in the air, and when the temperature was too low-the absorption of oxygen being always in a direct ratio to the temperature; and under both conditions the cessation of the absorption of oxygen was followed by the arrest of the circulation. Viewing the Circulation in connection with Respiration, * TENNYSON. CIRCULATION AND RESPIRATION. 279 we see many arguments favourable to Professor Draper's hypothesis; but that there are some difficulties not easily reconcilable with that hypothesis, cannot be denied. For the present, however, it is enough to have mooted the question, and to have touched on some of the difficulties in the way of our accepting the heart as the sole agent in propelling the blood. "The relation between the interspaces of the capillaries, and the blood thus introduced to them, continues the current. The oxidising arterial blood has a high affinity for those portions that have become wasted; it effects their disintegration, and then its affinity is lost. The various tissues require repair; they have an affinity for one or other of the constituents of the blood; they take the material they need, and their affinity is satisfied; or secreting cells originate a drain upon the blood, and the moment they have removed from it the substance to be secreted, they have no longer any relation with it. So processes of oxidation, of nutrition and secretion, all conspire to draw the current onwards from the arteries, and push it towards the veins."* We have now brought to a close our survey of the course and cause of the Circulation, and assigned to each labourer in this difficult field of research his share in the work. As an episode in the History of Science, the discovery of the circulation will always command the interest of readers; and if the foregoing sketch has had the good fortune to secure the attention of any medical readers, we hope it may have the further effect of inducing them to go carefully through the immortal works of WILLIAM HARVEY. * DRAPER, p. 145. + HARVEY'S works have been ably translated by Dr. R. WILLIS, and published in one volume by the Sydenham Society. Several times in the course of our exposition we have been forced to allude to the passage of the blood through the lungs, and the changes there impressed on it. Indeed, without such changes blood could serve no purpose in the organism. Respiration is the mainspring of animal exist In the next chapter we shall attentively examine it. ence. CHAPTER VI. RESPIRATION AND SUFFOCATION. Two suicides-Suffocation of Seventy-two persons on board the "Londonderry -History of our knowledge of respiration-The air we breathe-Distinction between respiration as an Animal Function and as a Property of Tissue-Oxygen the life-giver Varnished eggs will not develop; tail of a tadpole developing after separation from the body-The breathing-mechanism in various animals -The process of breathing-Tight-lacing-Alterations of the air in respiration -Necessity of ventilation-German taverns-How the organism accustoms itself to bad air-Effect of bad air in depressing the vital functions-Respiration a vital, not a physical, problem-Suffocation: various forms of-Carbonic acid not a poison-Oxide of carbon a poison: its effects on the blood-Deaths by suffocation: escape of gas: drowning-Respiration not a process of oxidationThe balance of nature-Trees not beneficial in streets-Varieties of animal respiration-Respiration during sleep-Effect of temperature on respiration-Why do we breathe? A FEW years ago a young Frenchman, named Déal, finding his hopes of making a figure in the world were daily becoming more chimerical, resolved to die; and that he might not quit the world without producing some "sensation," he left this written account of his dying moments :"I have thought it useful, in the interest of science," he wrote, "to make known the effects of charcoal upon man. I place a lamp, a candle, and a watch on my table, and commence the ceremony. It is a quarter past 10; I have just lighted the stove; the charcoal burns feebly. "Twenty minutes past 10: the pulse is calm, and beats at its usual rate. 66 Thirty minutes past 10: a thick vapour gradually |