vi PRACTICAL DIRECTIONS 101 



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 for- 

 wards, 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 preparation, 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 water 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). 



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

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



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



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



the head as the carotid artery, having at its origin the carotid labyrinth. 



