III.] HUMAN BLOOD. 119 



In the more expensive forms of warm stages, as those of M. 

 Schult/e and others, warm water at a known temperature is passed 

 through a brass box which rests on the stage of the microscope, and 

 the exact temperature is determined by means of a delicate thermo- 

 meter. Fig. 76 shows a convenient form of hot stage, which can be 

 clamped to the stage of any microscope. It is heated by means of 

 warm water which passes in at B, and, after traversing a system of 

 tubes, out at B'. It is provided with a thermometer. 



6. Crenation of Coloured Corpuscles (H). Mix a drop of 

 human blood with a 2 per cent, solution of common salt. Note 

 the change of colour. (a.) Observe that 



some corpuscles shrink in part, and become 

 crenate or beset with short spines (fig. 77). 

 This is due to exosmosis of fluid from the 

 corpuscles. The colour becomes slightly 

 deeper than in normal corpuscles. All the 

 corpuscles are not affected simultaneously 

 or to the same extent. 



In some individuals, merely exposing the 

 blood to the air for a few minutes before 

 applying a cover-glass suffices to produce 

 this condition (fig. 72, e, f) ; but in any 

 specimen of blood it may be readily produced in the majority of 

 red corpuscles by acting on them with a saline solution of appro- 

 priate concentration. 



It has been observed that in the blood of a mammal poisoned by 

 Calabar-bean the blood-corpuscles are crenated. 



7. Dilute Alkalies. Use a 0.2 per cent, solution of caustic 

 potash (i.e., 2 grams in 1000 cc. of normal saline). It dissolves 

 both the red and white corpuscles. 



8. Fibrin (H). Make a preparation of human blood, using a 

 large, drop. Cover it and put it aside for half-an-hour or longer 

 until the blood coagulates. 



(ft.) Observe carefully, and in the meshes between the rouleaux 

 fine threads forming a delicate network will be seen. They are 

 fibrils of fibrin. 



(b.) A better method, however, is to mix on a slide a drop of 

 blood with a drop of normal saline solution. Cover and put it 

 aside for an hour or so to clot, and then irrigate with water or dilute 

 alcohol (p. 25), which rapidly decolorises and washes away the red 

 corpuscles, and thus brings into view a fine fibrillar network of fibrin 

 in the field. Irrigate with a watery solution of Spiller's purple (r 

 per cent.), which stains the network of fibrin a purplish tinge (fig. 

 78). Raise the cover-glass, and to it will be found adhering a thin 

 film of fibrin. Dry the film, apply a drop of balsam, and mount the 



