300 THE BIOLOGY OF THE FROG chap. 



point of origin. They are found embedded along the 

 course of certain nerves such as the vagus and facial ; in 

 the walls of the intestine {AuerbacJVs and Meissner's 

 plexuses) ; in the heart (Remak's, Bidder's, DogiePs ganglia 

 and several smaller groups of cells) ; in the walls of the 

 bladder ; and in the skin. 



Reflex Action. — The reflex actions of the spinal cord of 

 the frog may be illustrated by the following experiment: 

 Cut the spinal cord of a frog across just behind the medulla 

 and, after destroying the brain, suspend the animal upon 

 a hook passed through the upper jaw. Now pinch one of 

 the toes ; the hind foot will be drawn up to the body. If a 

 piece of blotting paper saturated with dilute acetic acid be 

 placed on one side of the body, the hind leg of that side 

 will be brought forward and the acid wiped away with the 

 foot. If the acid is placed near the middle of the body, 

 both hind feet may be employed to remove it. The vigor 

 of the response depends upon the strength of the stimulus, 

 a weak stimulus producing only a slight movement, while a 

 very strong stimulus will throw the animal into violent con- 

 tortions. Similar responses may be evoked from the fore 

 limbs, and if acid be placed on the side of the body some 

 distance in front of the hind legs, both fore and hind limbs 

 may be employed to remove the irritant. These actions are 

 of a very definite and mechanical type, and, if the frog is in 

 good condition, follow inevitably upon the application of the 

 stimulus. They are termed reflex actions on account of a 

 certain analogy with the reflection of light which may be 

 thrown back upon its original source by a mirror. The 

 spinal cord nerves contain two kinds of fibers, the afferent 

 or sensory, which conduct impulses from the periphery to 

 the cord, and efferent (usually motor fibers), by means 

 of which impulses are carried outward to other organs. 



