vi INJECTION OF BLOOD-VESSELS 99 



c. Now turn the frog the other way upwards, pin it down 

 in the dissecting-dish, and open the body-cavity as before 

 (p. 32), taking great care not to cut the abdominal, musculo- 

 cutaneous, and other veins. Slit open the pericardium and 

 remove as much of it as possible, so as to expose the entire 

 heart. The structure of the heart and the course of many of 

 the blood-vessels can also be made out in the specimen from 

 which you have already removed the alimentary canal. In 

 the following dissections, use a dissecting lens whenever 

 necessary. 



I. In the heart (Figs. 3, 4, 7, 20 and 21), notice again the 

 ventricle, and the right and left auricles (appearing single in 

 the entire heart), and make out also the conus arteriosus, 

 dividing into two distally, and the sinus venosus (dorsal). 



If the heart is still beating, notice the order of contraction 

 of its different chambers (p. go). 



Injection of the Arteries. The tracing of the arteries is 

 greatly facilitated by filling them with some coloured sub- 

 stance. The operation requires, therefore, a coloured fluid 

 or injection-mass capable of traversing the arteries, and some 

 contrivance by which it can be injected into them. 



The most convenient injection-mass is made as follows : 



1. Grind up in a mortar 4 grammes of " French blue " (to 

 be had at the oilman's) with 4 cubic centimetres of glycerine 

 and the same quantity of methylated spirit. 



2. Grind up 50 grammes of common laundry-starch, with 

 50 cubic centimetres of water and 25 of methylated spirit, 

 and add to the mixture the colour prepared as above. Mix 

 thoroughly and strain through muslin. 



This injection-mass will keep for an indefinite period in a 

 stoppered bottle, requiring only to be stirred up when used. 

 If it is considered too troublesome to make, a simpler but 

 less satisfactory mass may be made by simply stirring up 

 some French blue in water in the proportion of a teaspoonful 

 to a tumblerful. 



For injecting the mass into the blood-vessels, the most 

 satisfactory instrument is a brass injecting syringe, holding 

 about one ounce, provided with nozzles of various sizes. 

 This is, however, expensive, and an ordinary glass syringe, 

 to be had of any druggist, will answer the purpose very 

 fairly if provided with a proper nozzle or cannula. This 

 latter is made by drawing out one end of a piece of glass 

 tubing about two inches long until it is fine enough to pass 

 into the conus arteriosus ; a piece of india-rubber tubing is 

 then used to connect it with the fixed nozzle of the syringe. 



H 2 



