GENERAL PHYSIOLOGY OF THE FROG 15 



where the nerves to the muscles of the hind legs originate; an impulse from this 

 level of the cord along the nerves in question to the particular muscles needed; 

 contraction of these muscles producing movements of the hind legs. 



In this and all subsequent experiments, make careful observations and take 

 notes on what happens and write up the experiment later in your notebook, 

 according to the plan suggested in the introduction. 



B. FUNCTION OF THE MUSCULAR SYSTEM; CONTRACTILITY 



Contractility is the capacity of living matter to shorten itself. It is probable 

 that all kinds of movements in animals are due to this property, which is par- 

 ticularly specialized in the muscles. 



1. Contractility of voluntary muscle (muscle under control of the will). 

 Remove the hook from the frog's jaw and now pith the spinal cord (^4). Lay 

 the frog back upward, make a circular incision through the skin completely 

 around the base of the thigh, and grasping the cut edge of the skin, completely 

 strip the skin from the hind leg. On the dorsal side of the thigh, three muscles 

 will be seen: laterally, the glutaeus magnus; medially, the semimembranosus; 

 and between them the small slender ileo-fibularis. On the back of the shank is 

 a large spindle-shaped muscle, the gastrocnemius. Carefully separate the ileo- 

 fibularis from the semimembranosus and locate between them the sciatic nerve, 

 appearing as a stout, white cord running alongside of a dgrk-colored blood vessel. 

 Carefully isolate the nerve from the blood vessel, handling it with the utmost 

 care. Lift it up and while watching the gastrocnemius muscle cut through the 

 nerve. What happens? What is the stimulus? How does the stimulus get 

 to the muscle? How does the reaction time compare with that in the preceding 

 experiment? Why? The experiment may be repeated as many times as desired 

 by again cutting the nerve between the first cut and the muscle. Satisfy your- 

 self that in the motion the muscle actually becomes shorter and thicker, and that 

 this change in its shape is the cause of the movement. 



2. Contractility of involuntary muscle (muscle not under control of the will, 

 found mainly in the walls of the digestive tract). Turn the frog ventral side 

 upward and cut through the ventral body wall to the left of the median line from 

 the pelvic girdle up through the pectoral girdle. Gently pull the stomach out 

 so that it will be clearly exposed and with a forceps pinch the wall of the stomach. 

 Wait for the response. How does the reaction time compare with that of volun- 

 tary muscle ? Watch the contraction travel. In which direction does it go ? 

 This kind of contraction is called peristalsis. What is its purpose? 



3. Contractility of heart muscle. Free the heart carefully from the peri- 

 cardial sac and observe that the beating of the heart is nothing but a rhythmical 

 contraction of its muscular walls. In what order do the parts of the heart beat ? 

 Observe changes in color, form, and size of the auricles and ventricle during 



