5 2 PRACTICAL HISTOLOGY. 



CIRCULATION OF THE BLOOD. 



The circulation of the blood may be studied in any vascular transparent organ, e.g. web of 

 frog's foot, lung of frog, and also in the mesentery. For the student it is best to study the 

 circulation in the web of a frog's foot. 



METHOD. Select a frog with a slightly pigmented web, and with a hypodermic syringe 

 inject into the lymph-sac under the skin of the back four minims of a quarter per cent, watery 

 solution of curara, and place the frog under a bell jar till its motor nerves are paralysed, i.e. till 

 it ceases to move when it is pinched. Place the frog on a piece of card-board one and a-half 

 inches wide and six long, with a triangular slit of one-inch base cut at one end of the card. Tie 

 a thread round the tips of two adjoining toes, e.g. the third and fourth. Fix the threads in a 

 slit made in each horn of the cardboard, and stretch the web gently across the slit. Moisten 

 the web, place the card-board on the stage of the microscope, and fix it with clips. 



EXAMINATION (L). Observe the arteries, with the blood moving in them from the large to 

 the smaller vessels. In the centre of the stream a more rapid current, and between it and the 

 wall of the vessel the slower stream or lymph-space. Select a vein, known by its thinner walls 

 and the direction of the blood-stream, and notice the movement from the smaller to the larger 

 vessels, and the stream slower than in the arteries. Next observe the network of capillaries, 

 with the blood-corpuscles moving in single file. Notice also the pigment-cells, some of them 

 branched ; but if they are contracted they look like small black specks. Cover the web with a 

 fragment of a cover-glass. (H). Study a capillary ; observe its thin wall and the. passage of 

 the corpuscles in single file through it. The coloured corpuscles are elastic, as shown by the 

 way in which they twist and easily become distorted, and as easily regain their normal shape. 

 Select a small vein and observe the colourless corpuscles dragging lazily along in the lymph- 

 space and adhering to the wall of the vessel. 



If desired, the phenomena of inflammation of the web can easily be studied by applying a 

 small quantity of mustard moistened with water, or some other irritant, for a few minutes. 

 Wash off the irritant from the web and study the effects. 



