XIII.] THE FROG. 229 



7. The dorsal aorta and its branches. 



a. Remove the kidneys with vena cava inferior and 

 the generative organs: the dorsal aorta is then 

 laid bare lying on the bodies of the vertebrse. 



b. Follow the systemic aortce (5. a. /3) round the 

 neck; they will be found to unite beneath the 

 vertebral column to form the dorsal aorta. 



c. Follow the aorta backwards: it gives off many 

 branches on its course; note the large one (cwliaco- 

 mesenteric artery) arising from it just below its 

 point of formation. 



d. Small branches to the renal and generative 

 organs (only the cut ends of these branches can 

 now be foimd) and to the muscles of the back. 



e. Near the pelvis it ends by dividing into two 

 trunks (the iliac arteries) which run behind the 

 pelvic bones, giving off hypogastric branches to 

 the bladder and the walls of the abdomen. 



f. Turn the animal over on to its belly and trace 

 the iliac arteries backwards: they are mainly 

 continued down the thigh as the femoral arteries. 



8. The pulmonary veins. 



a. Trace them from the left auricle to the lungs. 

 Examine the left auricle carefully and find the 

 opening of the common pulmonary vein into it. 



G. The nervous system of the frog. 



1. The method of exposing the hrain and spinal cord. — 

 Take a frog which has been a day or two in spirit ; 

 divide the skin along the middle of the dorsal sur- 

 face from the snout to the anus and reflect it to 



