THE BLOOD. 



77 



Action of Reagents. Considerable light has been thrown on the 

 physical and chemical constitution of red blood- cells by studying the 

 effects produced by mechanical means and by various reagents: the fol- 

 lowing is a brief summary of these reactions: 



Pressure. If the red blood-cells of a frog or man are gently squeezed, 

 they exhibit a wrinkling of the surface, which clearly indicates that there 

 is a superficial pellicle partly differentiated from the softer mass within; 

 airain, if a needle be rapidly drawn across a drop of blood, several cor- 

 puscles will be found cut in two, but this is not accompanied by any es- 

 cape of cell contents; the two halves, on the contrary, assume a rounded 

 form, proving clearly that the corpuscles are not mere membranous sacs 

 with fluid contents like fat-cells. 



Fluids. Water. When water is added gradually to frog's blood, the 

 oval disc-shaped corpuscles become spherical, and gradually discharge 

 their haemoglobin, a pale, transparent stroma being left behind; human 

 red blood-cells change from a discoidal to a spheroidal form, and dis- 

 charge their cell-contents, becoming quite transparent and all but invisible. 



Saline solution (dilute) produces no appreciable effect on the red 



FIG. 70. 



blood-cells of the frog. In the red blood-cells of man the discoid shape is 

 exchanged for a spherical one, with spinous projections, like a horse- 

 chestnut (Fig. 70). Their original forms can be at once restored by the 

 use of carbonic acid. 



Acetic acid (dilute) causes the nucleus of the red blood cells in the 

 frog to become more clearly defined; if the action is prolonged, the nu- 

 cleus becomes strongly granulated, and all the coloring matter seems to 

 be concentrated in it, the surrounding cell-substance and outline of the 

 cell becoming almost invisible; after a time the cells lose their color alto- 

 gether. The cells in the figure (Fig. 71) represent the successive stages of 

 the change. A similar loss of color occurs in the red cells of human blood, 

 which, however, from the absence of nuclei, seem to disappear entirely. 



Alkalies cause the red blood-cells to swell and finally disappear. 



Chloroform added to the red blood-cells of the frog causes them to 

 part with their haemoglobin; the stroma of the cells becomes gradually 

 broken up. A similar effect is produced on the human red blood-cell. 



Tannin. When a 2 per cent, solution of tannic acid is applied to 

 frog's blood it causes the appearance of a sharply-defined little knob, pro- 

 jecting from the free surface: the coloring matter- becomes at the same 

 time concentrated in the nucleus, which grows more distinct (Fig. 72). 



