PRACTICAL HISTOLOGY. 



[XIX. 



FIG. 220. Capillaries Injec 

 ted with Silver Nitrate. 



let the brain of the animal be destroyed. An hour before the web 

 is to be examined, place two or three drops of a . 5 per cent, watery 

 solution of curare in the lymph-sac under the skin of the animal's 

 back. The drug should not act too rapidly. After a hour or so it 

 paralyses the extremities of the motor nerves, 

 and thus makes the frog motionless. A small 

 dose of the drug is given in order not to 

 affect the calibre of the blood-vessels. 



Make a frog-plate by taking a piece of 

 stout cardboard or thin slip of wood 1 5 cm. 

 long (6 inches) and 5. cm. (2 inches) broad. 

 At one end of it cut a triangular slit whose 

 base is 2 cm. or less in width. Tie a thread 

 round the tip of, e.g., the third and fourth 

 toes of the hind-limb, place the frog on 

 its belly on the board, and by means of 

 the two threads gently stretch the web 

 across the triangular slit. It must not be 

 drawn too tightly. The threads can be fixed 

 in slits made in the horns bounding the 

 triangular aperture. To prevent evaporation from the frog, it had 

 better be placed in a moist cotton rag or surrounded with moist 

 blotting-paper. Moisten the web with a drop of water, and cover 

 it with a narrow fragment of a cover-glass. In selecting a frog, 

 choose a light-coloured one. 



(a.) (L and H) Find an artery, and note that the blood flows 

 from larger into smaller vessels with what appears to be consider- 

 able velocity. Contrast it with a vein, in which the blood flows 

 in an opposite direction, i.e., from smaller vessels capillaries to 

 larger ones, but the current is slower in the veins than in th 

 arteries. The walls of the vein are slightly thinner than those of 

 the artery. 



(b.) The capillaries, small and of uniform diameter, with the 

 corpuscles moving in single file. The flow is uniform. 



(c.) In the arteries and veins note the rapid red central stream, 

 or axial zone of coloured corpuscles, and next the wall, on either 

 side, the peripheral zone or space of Poiseuille, narrow and free 

 from red corpuscles, but containing a few white ones rolling lazily 

 along the vascular wall (H). 



(<i.) If a red corpuscle happen to be arrested at the bifurcation 

 of a capillary, other corpuscles impinge on it and bend it. As 

 soon, however, as it is dislodged it regains its shape, so that the 

 red corpuscles are highly elastic. 



(6.) Numerous pigment-cells, some of them contracted, others 

 expanded, are visible (fig. 144). 



