Page:Popular Science Monthly Volume 26.djvu/760

 decisive in the matter: A frog is suspended by the head, and his legs are allowed to dip into a vessel of dilute acid. After some time the irritation causes the legs to be removed. The average time is ascertained by frequent trials—then the animal's cord is cut just below the medulla. The time which now intervenes between contact with the acid and withdrawal of the limbs is much shortened, and the action is decidedly more vigorous. Setschenow's experiments (1863) show that this influence of the brain-centers can be greatly augmented by direct irritation of the optic thalami. The rule respecting the reflex activity of the cord would lead us to expect an increase of the activity upon an increase of stimuli. This is true in general, but Wundt has proved that the rule applies to those stimuli only that are carried to the same part of the cord. If an afferent nerve in some other portion of the body should be irritated simultaneously with the cord, reflex action would entirely cease.

We are now to consider certain activities of the cord which are the most remarkable of all its manifestations. I allude to the experiment first performed by Pflüger. This experiment has been frequently repeated and variously interpreted. Pflüger decapitated a frog, and then placed some acetic acid on the animal's thigh. This headless creature immediately wiped off the acid with the bottom of the foot

 (Sappey. after Hirschfeld.) 1.1. superior "vermiform process" (middle lobe) whose anterior extremity has been pushed backward in order to show the corpora quadrigemina; 2. posterior extremity of the superior and inferior "vermiform processes," and of the median fissure of the cerebellum; 3. great circumferential fissure; 4. great fissure of the upper surface which divides it into two principal segments; 5. posterior of these segments in the form of a crescent; 6, 6, 6, 6, 6, anterior segment, quadrilateral, and composed of five secondary curved segments like the preceding—each of these segments being composed of closely packed "laminæ" of different sizes, separated by fissures of varying depths; 7, 7. sections of the cerebral peduncles; 8. "posterior commissure" of the cerebrum; 9, corpora quadrigemina.

of the same side. Pflüger then removed this foot and again placed acid on the same thigh. The animal at first, as though deceived, endeavored to rub away the acid in the same way as before. This being impossible, the frog soon ceased trying that method, and seemed to be seeking out some other plan. Finally, he made use of the foot which