vi PRACTICAL DIRECTIONS 101 



enough to allow of the arteries "being 'conveniently 

 traced. 



Injection of the Veins. The veins are much more difficult 

 to inject than the arteries, but if you wish to make a double 

 injection on the same specimen, colour the injection-mass 

 with vermilion or carmine in the case of the arteries instead 

 of with French blue, using the latter for the veins. The 

 operation is best performed by inserting the nozzle into an 

 incision in the abdominal vein (by directing the nozzle 

 forwards, the portal vein will be injected : by directing it 

 backwards the pelvic and renal portal veins), and also into 

 one of the subclavians. But for a really satisfactory prepara- 

 tion, it is best to inject from the heart through the conus, 

 as directed above, with a weak, warm solution of gelatine 

 (in the proportion of one part of gelatine to two parts of 

 water), coloured with precipitated carmine. In this case 

 the injection-mass, containing only microscopic particles, 

 passes from the arteries through the capillaries into the veins, 

 keeping throughout to the course taken by the blood during 

 life, and therefore unimpeded by the valves of the veins. A 

 syringe must be used, since the medicine dropper will not 

 give sufficient pressure, and the animal should be placed in 

 warm water during the process. 



II. Now pin down under w r ater and make out the course 

 of the chief veins (p. 82, Fig. 21) : 



1. The two precavals, and the external jugular, internal 

 jugular, subclavian, and musculo-cutaneous. 



2. The postcaval, to see which turn the viscera on one side 

 (Figs. 3 and 4). Note the renal, spermatic or ovarian, and 

 hepatic veins. 



3. The hepatic portal vein and its factors. 



4. The abdominal vein and pelvic veins. 



5. The veins from the hind -legs can be more easily seen at 

 a later stage, after the alimentary canal is removed, and so 

 their examination is best left until certain of the arteries 

 have been traced (or use the specimen you have dissected 

 previously for this purpose). Remove the skin from the 

 thigh, place the frog on its side, and make out the femoral, 

 pelvic (already seen), renal portal, and sciatic veins, as well as 

 a large vein from the muscles of the back. 



6. The two pulmonary veins. 



Make a sketch of the heart and as many of the veins as you 

 have followed out up to this point, inserting the others after 

 removing the alimentary canal (see p. 102). 



