XIIL] THE FROG. 199 



and skull as is needful to lay open the cavity which 

 contains the cerebro-spinal nervous centres. Pin 

 the frog in a dissecting dish, on its right side, with 

 sufficient water to cover it, and study the position of 

 the various organs in relation to a median longitu- 

 dinal plane, making a careful diagram of the parts 

 displayed. 



/J 7. In a frog which has lain in bone-softening solution 

 (say i o/o chromic acid) sufficiently long to soften 

 the bones, make transverse sections (i) through 

 the eyes, (2) through the centres of the tympanic 

 membranes, (3) through the shoulder-girdle, (4) 

 through the hinder half of the abdomen. Compare 

 them with the foregoing dissection. 



B. DISSECTION OF THE VISCERA IN THE VENTRAL CAVITY. 



i. Lay a frog, which has been killed with chloroform, 

 on its back and pin it out on a layer of paraffin or 

 beeswax, under water; divide the skin along the 

 abdominal median line from the pelvis to the 

 front of the lower jaw; next make a transverse 

 incision at each end of the longitudinal one, and 

 then throw outwards the two flaps of skin thus 

 marked out. The following points may now be 

 noted. 



a. A great vein (musculo-cutantous] on the under- 

 surface of each flap of skin, about the level of 

 the shoulder. 



b. Some of the muscles of the abdominal wall, 

 covered by a thin aponeurosis: through this 

 latter can be seen 



a. The rectus abdominis running from pelvis to 



