196 INTRODUCTION TO GENERAL PHYSIOLOGY 



this way may be examined with a high power, after covering the 

 preparation. 



The urinary bladder of the frog exhibits the arrangement of 

 smooth muscle well. Distend it with alcohol by means of a pipette. 

 When hardened, cut out a piece of a few millimetres square. Stain. 

 Mount in glycerine. 



Contractions of the Frog's Stomach 



Cut with scissors a ring from the frog's stomach by two parallel 

 transverse cuts. Pass a bent pin through the ring and hang it up 

 on a rod over the lever used above for the heart (p. 187). Pass 

 through the ring another hook and connect it to the lever by a 

 thread. The muscle will usually at first be in tonic contraction. 

 Note that running warm (25 to 30 C.) 0.7 per cent, sodium chloride 

 over it causes relaxation. If the muscle has already relaxed some- 

 what, the first effect of the warm saline may be to excite a con- 

 traction, but this is followed by a marked relaxation. A series of 

 rhythmic contractions sometimes follows. 



The tonic contraction will slowly give way without warming, 

 especially if a small weight, I or 2 gm., be attached to the lever 

 so as to stretch the preparation slightly. After a time there may 

 be slow rhythmic contractions and relaxations. 



Apply induction shocks by twisting the end of a fine copper 

 wire, attached to one terminal of the secondary coil, around the 

 upper pin and a wire from the other terminal to the lower pin. 

 Note the slow contraction. 



Secretion 



Examine under the microscope a thin bit of the rabbit's 

 pancreas. Note the granules in the cells. 



Make sections of the salivary gland as described above for 

 intestinal mucous membrane. Study the general arrangement. 

 Vertical sections through the frog's skin show typical simple 

 glands. 



The disappearance of granules in the act of secretion may be 

 seen in the living stomach of the newt, as described by Langley 

 and Sewall (Journal of Physiology, vol. ii., p. 286). Feed a newt 

 with small earthworms or by introducing with a pipette some 

 diluted white of egg into the stomach. In twenty-four hours the 

 digestive process is over, the glands have assumed the resting 

 appearance and are ready for renewed secretion, being full of 

 granules. Take another newt three hours after feeding. The 

 granules in some of the cells have nearly disappeared ; in others 



