406 



BLOOD. 



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 (^j 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 

 315 75ffi5w tns of a line and one 367 7u ^ogths 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. His first observation, nevertheless, 

 comes extremely near what we are inclined to 

 regard as the truth (22 Tg L ro ths of a line); a se- 

 cond observation, however, gives a much smaller 

 diameter (.IS^Uw 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 



Vide, Some microscopical Observations on the 

 Blood, &c. in Philos. Mag. Aug. 1827. 



serum, and which are in fact very nearly of 

 the dimensions indicated.* 



Mr. Bauer and Sir E. Home had pre- 

 viously assigned SB-j^^tlis 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 glass.f 



Dr. VVollaston held that the globules of the 

 human blood did not exceed 20-, D ^ 5 ths of a line 

 in diameter, which is considerably different 

 from our mean ; and Dr. Young did not esti- 

 mate them at more than 16 i^oo tns ^ a line.J 

 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-makers 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 IQ-jg^jths, the se- 

 cond Sl-jj^ths 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 Sl^^lns, 

 or in vulgar fractions the g^th 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. 



* Vide Additions to the Croonian Lecture, 

 Philos. Trans. 1818. 



t Loc. cit. 



j Young, Elem. of Med. Literature, 8vo. 

 Lond. 1818. 



