VI PRACTICAL DIRECTIONS tot 



in the case of the arteries instead of with French bhie, 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 for- 

 wards, the portal vein will be injected : by directing it backwards 

 the pelvic and renal portal veins. Bui 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 pre- 

 cipitated carmine. In this c{ise the injection mass, containing only 

 microscopic particles, passes from the arteries through the capillaries 

 into the \eins, 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 make out the course of the chief vet'/is (p. 82, Fig. 21) : (if 

 not injected, put no water into the dissecting-dish at present) : — 



1. The Iwo Jtretava/Sf and the external jugular^ inieriial jugular^ 

 SHb-clavian, and miisculo-ciitaneoiis. 



2. Ihn 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 vein, 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 

 ahmentary canal (see p. 102). 



III. The chief arteries may now be followed out (p. 80, Fig. 20) : — 

 Note the carotid, the systemic, and the pulmo-cutancoiis trunk, arising 



from the conus arteriosus, and then trace each of these out as follows ; — 

 I. The carotid trunk gives off a lingual artery, and is continued into 

 the head as the carotid arterv, having at its origin the carotid ple.xus. 



