70 



THE AMEKICAN MONTHLY 



[April,^ 



has a strong tendency to solidify in 

 the form of reddish crystals, some- 

 times even within the disk itself. 

 Different forms have been recog- 

 nized in the blood of several differ- 

 ent kinds of animals, but they cannot 

 yet be considered absolutely distinc- 

 tive, even of the very limited number 

 of animals where they seem to be dif- 

 ferent from others. 



The disks of nearly all the mam- 

 malia, including the human races, are 

 round and without a definite nucleus. 

 They are not perceptibly changed by 

 being preserved in a dry state for 

 several years. Measurements can be 

 made of the same objects by means of 

 an eye-piece micrometer, which shall 

 agree, on the average, within 1-300,000 

 of an inch or less. I have done this 

 repeatedly, and so have many others. 

 The measurements are most conveni- 

 ently made, and recorded in micro- 

 millimetres carried to one place of 

 decimals. Measured thus, the hu- 

 man disks range, as given by a great 

 number of modern authorities, be- 

 tween about 7.1 and 7.9 micromilli- 

 metres, the average being about 7.5. 

 The average of many thousands of 

 measurements that I have made is 

 about 7.4. The various human races 

 are practically alike, including the 

 Negro, the recorded measurements 

 varying about o.i or 0.2, with no 

 probability that the difference is con- 

 stant. The blood of various mon- 

 keys is very near that of man, with 

 probably no distinctive difference of 

 size. That of the dog has smaller 

 disTcs, perhaps averaging 6.9, yet no 

 person of good judgment would 

 claim to distinguish it infallibly from 

 that of man, which averages over 7.4. 

 Not that there is any difficulty in at- 

 taining that degree of precision in 

 measurement, or much more if re- 

 quired, or in eliminating accidental 

 errors by averaging from a very large 

 number of measurements ; but be- 

 cause the normal size of the disks of 

 the dog is so near that of man that 

 the series may overlap, and there 

 may be some dogs whose disks, if 



every one were measured, would bell 

 equal in size to those of some men. 

 Very different is the case of the pig 

 and ox, whose disks measure about 

 6.0 or 5.9, the horse at 5.5, and the 

 sheep at 4.8. These figures may be 

 large, but there is no probability that 

 any revision will raise them near to 

 those of man, or that a single sheep, 

 for instance, lives whose disks would 

 measure 7.0 and upwards. It is, 

 therefore, safe to say that under 

 favorable circumstances, including 

 the opportunity to examine a large 

 number of characteristic and well- 

 formed disks, the blood of mutton, 

 and probably of any animal whose 

 disks are not larger than those of the 

 pig, can be distinguished from that 

 of men. This is really all that has 

 been claimed by Dr. J. G. Richard- 

 son and others, who have claimed 

 decisive value for this method of dis- 

 criminations. That dog's blood could 

 be thus distinguished, has never been 

 prominently claimed or generally be- 

 lieved ; and that any one kind of 

 blood could be, by such means, abso- 

 lutely and unqualifiedly identified, as 

 distinguished from all other kinds, is 

 simply absurd and claimed by nobody. 



Such measurements should be made 

 with powers of not less than 1,000 

 diameters ; 2,000 or 3,000 being pre- 

 ferable. By the use of amplifiers or 

 high oculars, such powers may be at- 

 tained by almost any microscope ; 

 but there are few instruments that 

 can be worked at such powers to ad- 

 vantage, and the successful use of 

 such powers requires great tact and 

 judgment as well as experience in the 

 manipulator. 



The quantity of blood required for 

 this method of analysis is really very 

 small. It has been computed that 

 there are about 5,000,000 of the red 

 disks to one cubic millimetre of blood,^ 

 say the bulk of a common pin-head, 

 and that the weight of each is about 

 0.00008 of a milligram, so that you- 

 could measure 75 or 80 disks without 

 exceeding the use of i-iooooth of a 

 grain of blood. 



