THE FROG 57 
high power ; the axis cylinder of each fiber appears as a dark spot 
surrounded by a clear space occupied by the medullary substance. 
The nervous elements of the cord are bound together by stellate 
neuroglia cells and by processes which arise from the tapering 
outer ends of the ependyma cells; these processes branch re- 
peatedly and some of them extend to the periphery of the cord. 
Occasional capillaries may appear in the sections. 
The nerve cord is closely surrounded by the following cover- 
ings: (@) an outer firm membrane, comprising the inner layer of 
the dura mater; and (0b) an inner vascular membrane, cor- 
responding to both the arachnoid and the pia mater of human 
anatomy. Both membranes are sometimes torn away in making 
the preparation. 
Under low power, draw a cross-section of the cord. The 
drawing should be about 12 centimeters in diameter. 
Be sure you understand the paths into and through the cord of 
“an impulse involved in a reflex act (see below). 
C. Reflex Action.—This experiment will be performed by an 
instructor. Kill a frog by cutting across the spinal cord between 
the skull and the first vertebra; then cut off the entire head except 
the lower jaw. Suspend the animal by the lower jaw so that the 
hind legs hang free. Pinch one of the toes and note that the leg 
is withdrawn. The nerve impulse set up by the irritation of the 
foot travels along sensory nerve fibers (i. e., fibers which termi- 
nate in sense organs) to the cord where it starts a motor impulse 
along the nerves innervating the muscles of the leg, causing them 
to contract. Such a direct response to the stimulus is called 
reflex action. Touch the foot with a hot needle, or with a bit of 
absorbent paper moistened with dilute acid, and observe the 
result. Apply dilute acid to the side of the body and see if the 
foot is used to wipe the acid away. Cut open the body cavity of 
the frog and lay bare the large nerves going to the hind legs. 
Irritate one of these nerves and observe the result. Cut one of 
these nerves across, irritate the end that is in connection with 
the leg, and observe the effect. 
