THE BLOOD. 95 



difficult to obtain in bulk, a specimen may be easily made for the micro- 

 scope in the following way: A small drop of dried blood is finely powdered 

 with a few crystals of common salt on a glass slide, and spread out; a cover 

 <rlass is then placed upon it, and glacial acetic acid added by means of a 

 capillary pipette. The blood at once turns of a brownish color. The slide 

 is then heated, and the acid mixture evaporated to dryness at a high tem- 

 perature. The excess of salt is washed away with water from the dried 

 residue, and the specimen may then be mounted. A large number of 

 small, dark, reddish black crystals of a rhombic shape, sometimes ar- 

 ranged in bundles, will be seen if the slide be subjected to microscopic 

 examination. 



The formation of these haemin crystals is of great interest and impor- 

 tance from a medico-legal point of view, as it constitutes the most cer- 



FIG. 84. Haematoidin crystals. (Frey.) FIG. 85. Hsemin crystals. (Frey.) 



tain and delicate test we have for the presence of blood (not of necessity 

 the blood of man) in a stain on clothes, etc. It exceeds in delicacy even 

 the spectroscopic test. 



Estimation of Haemoglobin. The most exact method is by the 

 estimation of the amount of iron in a given specimen of blood, but as this 

 is a somewhat complicated process, a method has been proposed which, 

 though not so exact, has the advantage of simplicity. This consists in 

 comparing the color of a given small amount of diluted blood with gly- 

 cerine jelly tinted with carmine and .picrocarmine to represent a standard 

 solution of blood diluted one hundred times. The amount of dilution 

 which the given blood requires will thus approximately represent the 

 quantity of haemoglobin it contains. (Gowers.) 



Distribution of Haemoglobin. In connection with the ascertained 

 function of haemoglobin as the great oxygen-carrier, the following facts 

 with regard to its distribution are of importance. 



It occurs not only in the red blood-cells of all Vertebrata (except one 

 fish (leptocephalus) whose blood-cells are all colorless), but also in similar 

 cells in many Worms: moreover, it is found diffused in the vascular fluid 

 of some other worms and certain Crustacea; it also occurs in all the striated 

 muscles of Mammals and Birds. It is generally absent from unstriated 



