BAUER has put a restraint upon himself, and for a time laid aside the prosecution of the Anatomy of Vegetables, which from his early youth has been his favourite occupation, to assist in bringing to light appearances in the anatomy of animals, which, without his aid, must still have remained in obscurity.

XII. Some additions to the Croonian Lecture, on the changes the blood undergoes in the act of coagulation. By Sir EVERARD HOME, Bart. V.P.R.S.

Read March 5, 1818.

SEVERAL of my friends, much more deeply versed in mathematics than myself, who were present at the reading of the Croonian Lecture, remarked that no spherical bodies could be accurately measured by the common micrometer, and therefore no correct idea of the diameter of a globule of the blood could be obtained by that means. They were also led to doubt the appearance represented in the coagulum, being real, since air, in all ordinary circumstances, when let loose, forms itself into globules, not moving in straight or curved lines.

These objections, coming from philosophers for whose opinions on such subjects I have the highest respect, induced me to request permission of the President to withdraw the Lecture, that I might correct any errors I had fallen into before the Paper came before the Committee. I found also upon reflection, that I had left the investigation more imperfect than I was aware of, since it is of very little consequence whether, in the act of drying, coagulated blood puts on this particular appearance or not, if I cannot at the same time. adduce proofs of the same changes taking place in coagula while they are still moist, and also in the blood when it is Bb

MDCCCXVIII.

extravasated, and coagulates in the interior parts of living animals.

I have therefore reconsidered this subject, and with the assistance of Mr. BAUER, have instituted a series of experiments, and had drawings taken to elucidate their results, which I hope will do away the objections made against coagulated blood having channels formed through it by the extrication of carbonic acid gas; and prove, not only that the same change takes place when blood is extravasated in living animal bodies, but that these channels have a communication opened between them and the neighbouring arteries, and that the fluid blood circulates through the channels in the coagulum. In laying these experiments and drawings before the Society, I request that I may be indulged in adding them to the Croonian Lecture; and with these additions, that it may be submitted to the judgment of the Council.

As the measurement of spherical bodies is a subject on which I am totally unfit to form an opinion, I requested my friend Captain KATER to have the goodness to measure the diameter of a globule of the blood, in what appeared to him the most satisfactory manner, and to explain to me the mode of doing it. He very readily complied with my request, and the following is the mode which he adopted.

A ruler divided into inches and tenths was placed on the box which supports the microscope, a mother of pearl miGrometer scale was placed under the microscope, each division of which was equal to one two-hundredth of an inch: viewing this with both eyes open, its image appeared to be projected on the ruler, and one division appeared to subtend the space of one inch. The micrometer scale being removed, blood

sufficiently dilute was placed under the microscope, and being viewed with both eyes open, a globule of blood appeared to occupy, in the first experiment, one half of one tenth of an inch, and in the second experiment, one third of one tenth of an inch upon the ruler. Hence the size of the globule by the first experiment will be equal to

I

of of 20% of 1 inch = 400 of an inch;

and by the second experiment

of of 0 of 1 inch

ΤΟ 200 = 60% of an inch;

the mean of which, or of an inch, may be considered as about the mean diameter of a globule of the blood.

This measurement of Captain KATER's, as it was natural to expect, corresponds with that which has been made by Dr. WOLLASTON, by means of a very ingenious micrometer of his own invention, a description of which has a place in the Philosophical Transactions; and with the measurement of Dr. YOUNG in his eirometer, of which he has given an account in his Introduction to Medical Education.

The diameter of a globule of the blood, measured by mathematicians of such eminence, is to be set down as 000 part of an inch; and the diameter in the micrometer, measured with all the accuracy that instrument is capable of, since such was the smallest apparent dimension which occurred in Mr. BAUER's experiments, as part of an inch.

I have taken more pains to have the difference between the measurement of a globule in these different modes ascertained, than the subject would appear to require; but its being known, will enable microscopical observers, unskilled in the higher branches of mathematics, to pursue their observations upon globules of different sizes, and continue to compare their re

lative size in the micrometer, which will give a correct result which ever mode of measurement is adopted respecting the original globule.

To do away the objection which has been made to gas being contained in the net-work formed in coagulated blood, I first made the following experiment. I placed a vessel nearly filled with blood drawn from the arm, under the receiver of an air pump, and by exhaustion extracted the gas contained in the blood. This blood deprived of its gas when coagulated, exhibited no appearance of net-work. In that part which had coagulated before the exhaustion was completed, the net-work was beautifully distinct.

When blood is drawn from the arm into a cup, and allowed to stand 48 hours, the serum is separated, and every where encloses the coagulum. The greater part of the surface of the coagulum is covered with small round holes in which the gas had collected, and then forced its way out into the serum. But if blood taken by cupping is allowed to stand 48 hours in a cup, sometimes the serum is only separated in small quantity, and does not rise above the coagulum, in consequence of a film or pellicle forming on the surface of the coagulum, and fixing itself to the edge of the cup all round. This pellicle when examined at the end of 48 hours appears to contain ramifying vessels. This arises from the mode by which the blood is extracted depriving it of a part of its carbonic acid gas, and what remains is not sufficient in quantity to burst the pellicle, and when in the act of extrication it arrives immediately under the pellicle, it is forced to spread in different directions, putting on this appearance.

Having ascertained that this appearance is produced by the