238 ELEMENTARY BIOLOGY. [CHAP. 



d. Small branches to the renal and generative 

 organs (only the cut ends of these branches can 

 now be found) 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. 



/ Turn the animal over on to its belly and trace 

 the iliac arteries backwards: they are mainly 

 continued down the thigh as the femoral arte- 

 ries. 



7- 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. 



i. The method of exposing the brain 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 

 each side, noting the small nerves running into it 

 on each side of the middle line ; remove the muscles 

 lying on the arches of the vertebra; open the neural 

 canal by dividing the membrane between the atlas 

 and occiput; then introduce one blade of a small, 

 but strong, pair of scissors into the cranial cavity, 

 and cut away bit by bit the bones which form the 

 roof of the skull, taking care that the point of the 

 scissors does not injure the brain. Next remove 

 the upper part of the arches of the vertebrae in a 

 similar manner. A delicate pigmented membrane 

 (the pia-mater) is now laid bare, covering the brain ; 



