Amphibia. 91 



be held between the thumb and finger while the nerve is pinched 

 (and the scissors are the surest pinchers), it will be felt to harden. 



This experiment should be repeated, varying the weight, until it 

 is made very clear that when the nerve is stimulated the muscle 

 shortens (which is the most important fact about the action), 

 thickens, and grows harder. 



Observe the thin, transparent membrane covering the muscle, 

 the muscle sheath. Tear the muscle to pieces, and note its fibrous 

 structure. Put a bit of the muscle in a drop of water on a slide, 

 and cover with a cover slip ; examine first with a low, and then 

 with a high, power, to see the cross markings of its finest fibers. 

 This kind of muscle is called striped or striated. 



PREPARATION OF A FROG'S SKELETON. 



Remove the skin and all the soft tissues. If you have a speci- 

 men that has been used for dissection, of course the anterior limbs 

 will come apart where the breastbone was severed. Remove the 

 soft tissues mainly by means of the scissors, being careful not to 

 cut too close to the joints. If the work is not completed at once, 

 return the specimen to the water. Renew the water frequently 

 and do not allow it to "get bad." If the ligaments decay, the 

 bones will fall asunder and make great difficulty on account of 

 their number and smallness. Do not boil the skeleton, or put it 

 into alkali. These processes may be useful for larger skeletons, 

 but are not good for such small ones as that of the frog. 



MOUNTING A FROG'S SKELETON. 



Get a piece of stiff, dark cardboard about six by eight inches. 

 Before mounting the skeleton on the card, get it nearly dry by 

 placing it on blotting paper or on cloth. If it is too dry, it will be 

 brittle ; if too wet, it will stain the card. Draw a fine pencil line 

 along the middle of the card. Place the skeleton along this line 

 and double up the limbs as if the frog were in the resting position. 

 A block of cork must be placed under the anterior end of the 

 spinal column to hold it up to the level of the skull. A small 



