ORIGINAL LECTURES.

LECTURES ON THE

BLOOD OF VERTEBRATA.

DELIVERED AT THE

Royal College of Surgeons of England,

DURING THE SESSION 1861-62.

By GEORGE GULLIVER, F.R.S. Professor of Comparative Anatomy and Physiology to the College.

LECTURE VIII.-Colour of the Blood-Discordant Observations on the Colour of Arterial and Venous Blood reconciled by Effects of Temperature-Causes of the Bright and Dark Colour of Blood-Uses of the Red Corpuscles.

Colour of the Blood.-We now proceed to consider the effects of the red corpuscles on the colour of the blood, as introductory to the interesting inquiry as to their agency in vivifying it and the parts through which they circulate, as well as their use in connexion with respiration and the production of animal heat. When the corpuscles are seen singly as transparent objects under the microscope, they are of a pale straw colour, of a redder hue when a few of them are so aggregated as to allow of less light passing through them, and of a darker colour when still more clumped together. Newton observed that blood is one of those fluids which is yellow when viewed in very thin slices. To the naked eye the colour is so well known, that "as red as blood" is a popular phrase; and no physiologist can be ignorant that this colour is owing, in vertebrates, to the red corpuscles, and is of a bright scarlet in arterial blood, and of a dark-red or Modena hue in venous blood. But the precise causes of this difference of colour, and the circumstances under which it increases, diminishes, or even ceases in the healthy animal, are by no means so well known.

Discordant Observations on the Colour of Arterial and Venous Blood reconciled by Effects of Temperature.-In the first place, as to the effect of temperature. The difference of colour is most marked in very cold weather, and least so in very hot weather; but having been always taught the striking floridness of arterial blood, and the darkness of venous blood, and having commonly witnessed this difference of colour in our own temperate climate, we seldom or never think of the possibility of any doubt or question as to this subject. Yet there have been very discordant observations, and by some of our best physiologists, as to the difference of colour between arterial and venous blood; and as the question is an important one, and even connected with the practice of our Profession, a few historical notices will be well calculated to fix this interesting point on our attention. Harvey believed that the colour of the two kinds of blood is essentially the same, though he admitted that there might be an accidental difference in the colour of the blood as it flows from the artery or vein of a living animal. But the question was a subject of controversy fifty years afterwards. Lower specially proved the difference of colour, and correctly inferred, from experimental inquiry, that the change of colour is produced in the lungs; and further supported this inference by observing that air produces just the same florid hue on the surface of the blood-clot out of the body-an observation which had also been made, two or three years before, by Fracassati, in a paper published in the Philosophical Transactions. Mayow fully admitted Lower's conclusions, and attributed the change of colour in the blood, during its course through the lungs, to its abstracting from the atmosphere a "nitro-aërial spirit," many of the properties of which, as he describes them, belong to oxygen. Dr. William Hunter, in his Lectures, as early as 1759, taught the true difference of colour between arterial and venous blood. Yet we find Haller, in 1756 and 1780, opposing the correct conclusions of Lower, and even asserting, that a hundred observations in dogs had convinced him that there is no difference of colour between the blood of the pulmonary artery and the blood of the pulmonary vein. But Priestly, who discovered his dephlogisticated air-our oxygen-in August, 1774, ascertained the correctness of Lower's results, and that air will act on the blood through a moist membrane; and he also proved that the brightened colour of the blood is produced only by the oxygen, and that VOL. I. 1863. No. 653.

carbonic acid, hydrogen, and azote have a contrary effect. Mr. Hunter removed the sternum from a dog, kept up the breathing of the animal by a bellows, and saw the blood acquire the scarlet colour in passing through the lungs. Then, at last, we have Dr. Davy making observations on sheep, at Malta, in the summer of 1829, when he carefully compared the blood of the jugular vein and of the carotid artery, and could see no difference whatever in their colour; in each it was less florid than the arterial blood of the same animal in an English winter, and less dark than the venous blood, being of a hue between the two. And this observation was so carefully conducted, that there could have been no mistake. In explanation, he supposes, that the higher the atmospheric temperature, and, consequently, the less need for the production of heat within the animal, the less difference there will be between arterial and venous blood, and the less power the venous blood will have of combining with oxygen, and of forming or evolving carbonic acid. Now, it it is remarkable that Dr. Crawford's experiments, upwards of fifty years before, lead to the same opinion. He put a dog for more than half-an-hour into warm water, and then found it difficult to distinguish the arterial from venous blood.

Here, then, is a series of facts not without value either in a physiological or in a practical point of view. The former has just been mentioned; and as to the latter, we must be very cautious in judging from the colour alone during hot weather, when the temperature of the atmosphere is upwards of 80°, of whether blood flowing from a wound be arterial or venous. Very likely Harvey's experiments were made at the hot season in Italy, and that Haller's took place during a warm summer at Lausanne.

Causes of the Bright and Dark Colour of Blood.-And now, having discussed the effect of temperature on the colour of the blood in the living body, we come to consider by what agent and how this effect is produced in the blood both in and out of the body. It is well known that blood is darkened even by a very brief stagnation both in living arteries and veins; and I have often been amused at the trick by which scheming farriers show "the blood as black as pitch," in order to prove their sagacity in having let out "that bad blood." No doubt, the general opinion, that the florid colour of arterial blood is due to the action of oxygen, according to the old views of Mayow, confirmed a hundred years afterwards by Priestly, is the correct one; but, in 1835, this was controverted very ingeniously by Dr. Stephens. It had been long known that earthy and alkaline neutral salts will render dark venous blood florid; and he was led to the conclusion, that the florid colour of arterial blood is caused by the agency of the salts of the serum on the hematozine, and that oxygen changes the colour of blood from venous to arterial merely by removing the carbonic acid, which, we have already remarked, Dr. Priestly had long before proved to darken the colour of the blood, and Dr. Stephens considers as the cause of this dark hue in venous blood. These views were adopted by Dr. Turner in his work on Chemistry, and Mr. Hoffman was also favourable to them. Dr. Christison agitated atmospheric air with a mixture of serum and red corpuscles of blood, and always found that oxygen disappeared, while carbonic acid was produced, but, owing to the strong solvent power of serum on this acid, he believed that more of it was formed than appeared in the residual air. In all these experiments venous blood acquired a bright vermillion hue, and the florid colour of arterial blood was heightened. Dr. Davy obtained the same results as to the absorption of oxygen by the blood, and the consequent brightening of its colour; but in the residual air he could detect only a trace of carbonic acid, and none at all when, instead of atmospheric air, pure oxygen was agitated with the blood. He further observed that venous blood, when subjected to the air pump, did not acquire the arterial hue, even when carbonic acid was extracted; and yet that the florid colour was imparted to venous blood by agitating it with a mixture of oxygen and carbonic acid gases, although the blood certainly absorbed a much larger portion of the carbonic acid than of the oxygen.

These results are quite irreconcilable with the doctrine of Dr. Stephens, but agree well with the older views which he disputes. Dr. C. J. B. Williams gave some experiments to prove that the florid effect produced by oxygen and the salts is by their causing more light to be reflected through the colouring matter. Dr. Davy concludes that neutral salts brighten the blood by so separating the corpuscles that they reflect more light; that water, acids, and other agents