These estimates accord very nearly with the last admeasurements published by M. Dumas, and taken by a different method. He gives 31 ten thousandths of a line as the mean dia meter of a globule of the human blood accord ing to his latest observations.* The conclusions come to by Dr. Hodgkin and Mr. Lister are also very nearly the same, these observers estimating the diameter of the globule of the human blood at 33 ten thou sandths of a line. These dimensions exceed, it is true, the mean of the measurements I have taken, but they are still within the limits of the individual variations which I have en countered among these corpuscles ; and as the physiologists quoted do not say whether their estimate was made from the mean of a number of observations, or from the measure ment of only a few globules more apparent than the rest, it is impossible for me to deter mine whence this discrepancy in our conclu sions arises, whether from actual varieties, from the manner of proceeding in determining the magnifying power of the microscope, or from an error in taking the limits of the image projected by the camera lucida.* The observations made some twelve years ago by Messrs. Prevost and Dumas do not differ from the measurements already given. The diameters they then assigned to the glo bules of the blood, amounted to 33 ten thou sandths of a line (th of a millimetre); but the magnifying powers they at that time employed did not exceed 300, and consequently the difference between the diameter of a globule of a line and one of a line might fail to be detected ; further, the errors which arise from the determination, always somewhat arbitrary, of the limits of the image, are sufficient to explain such slight differences as occur in the results of these very delicate observations. We must also add that Messrs. Prevost and Dumas at this time made use of a method, much less accurate than the camera lucida, for taking the apparent diameters of objects under the microscope, causing the image seen in the instrument with the right eye to coincide with the divisions of a scale placed laterally under the left eye. We therefore believe our selves justified in the preference we accord to the more recent observations of these gentle men.
The late Captain Kater, at the request of Sir E. Home, also made some observations with a view to determine the diameter of the globules of human blood, taking his measure ments in the manner formerly employed by Messrs. Prevost and Dumas, but making use of a power not higher than 200, by which the chances of erroneous conclusions were greatly increased. IIis first observation, nevertheless, comes extremely near what we are inclined to regard as the truth of a line); a se cond observation,however,gives a much smaller diameter of a line), but it is possible that in this case the observer may have taken his measurements from a globule divested of its colouring matter, or perhaps from one of the albuminous globules which abound in the serum, and which are in fact very nearly of the dimensions indicated.* Mr. Bauer and Sir E. Home had pre viously assigned of a line as the di ameter of these globules ; but their obser vations having been made with the ordinary micrometer are necessarily defective, inasmuch as the globules placed upon this instrument, and the divisions drawn on its surface, can never be simultaneously in the focus of the object glass4 Dr. Wollaston held that the globules of the
human blood did not exceed of a line in diameter, which is considerably different from our mean ; and Dr. Young did not esti mate them at more than 16 of a line.t It is also possible that both of these eminent individuals have measured the central nuclei of globules divested of their vesicular envelope. The results just specified having, farther, been come to by the aid of the eriometer, an in strument which we have searched for in vain through all the instrument-makets and col lections of philosophical apparatus in Paris, and as we are altogether ignorant of the degree in which its indications may be relied on, we cannot discuss these conclusions with an adequate knowledge of the elements from which they are derived. As to the measure ments published long ago by Jurine, they are so discordant that no confidence can be placed in them ; the first diameter he assigned to the globules of the blood was the se cond of a line.
From all that has gone before, then, and particularly from those researches which have been conducted under circumstances the most favourable to accurate conclusions, we may assume the mean diameter of the globules of the human blood to be about the or in vulgar fractions the part of a line.
Messrs. Prevost and Dumas have given the dimensions of the globules of the blood of a great number of other vertebrate animals; in these observations they employed the same means of estimating the diameters as in their earliest researches on the size of the globules of the human blood, so that to me their valu ations appear to fall somewhat short of the truth. This slight presumed inaccuracy, how ever, scarcely detracts from the interest of the general results; for the measurements being all taken by the same means and therefore comparable one with another, are adequate to show in the clearest light the differences that occur in the dimensions of these corpuscles in different animals. The following is the table of admeasurements given by the physiologists quoted.
From my own observations I am inclined to think that the globules of the blood of the frog have a mean long diameter of about 96 ,1„ths of aline; but the individual differences observable among the several globules ranged between and of a line. In the blood of the water-newt ( Salaniandra crisiata) I have obtained in my measurements of the long diameters of the globules the following ex treme individual varieties : minimum of a line, maximum The outline of the globules in all the verte brate animals is extremely well defined ; but they are readily deformed or put out of shape. Even during life their pressure mutually, or the pressure they experience between the cur rents in which they move and the parietes of the vessels against which they are driven, suf (ices to alter their form ; they are then fre quently seen to become elongated, to bend, in a word, to alter their figure considerably ; but they are extremely elastic, and readily and soon resume their pristine state.