This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
1. Place a frog on the table: note that it sits up and breathes (as shown by the movements of its throat), and either stays still or Jumps around as it pleases; i.e., it has a Will of its own, and its actions cannot be predicted.
2. Etherize two frogs (note, p. 86), removing them from the etherized water the moment they become insensible. With strong sharp scissors cut off from one frog (a) all the head in front of the anterior margins of the tympanic membranes, in the other frog (b) remove the head along a line joining the posterior borders of the tympanic membranes. Place both frogs aside on a dish containing a little water for half an hour. The quantity of water should be such that while keeping the frogs moist it will not reach to the wounds.
8. The frog (a) which has lost its cerebral hemispheres, but retained its mid brain, cerebellum, and medulla oblongata, will be found after the above-stated time sitting up in a natural position, and breathing. Left to itself it will, however, never walk or jump; it shows no sign of possessing a will. Its heart continues to beat and its respiratory muscles to contract, but left alone it stays where it is. Turned upon its back it will regain its feet; and put into water it will swim: its muscles, and the nerves controlling them, are, therefore, quite able to act. The animal stays still not because the parts of its body necessary to produce movements are injured, but because it can no longer will a movement. Such a frog shows very well the dependence of volition upon the presence of the cerebral hemispheres.
4 The frog (b) will have had its whole brain removed. Its heart will continue to beat, but its breathing movements will cease, because the respiratory centre, which lies in the medulla oblongata, has been cut away. It will also lie down squat, instead of sitting up like a normal frog, because its most important muscle co-ordinating centres have been removed with the mid-brain and cerebellum. Left to itself the animal will, within half an hour of the removal of the head, pull up its hind legs into their natural position, but after this it will make no movement. It has no volition.
5. Such a frog can, however, perform many co-ordinated reflex actions, which may be illustrated as follows: (a) Pinch a toe; it will be pulled away, (b) Soak some blotting-paper in vinegar, and then cut the paper into small pieces about 1/8 inch square. Put these bits of paper on different regions of the frog's skin, dipping the animal in clean water after each application, to wash away the vinegar.
It will be found that the brainless creature moves its limbs so as to wipe away the acid paper placed on its skin. The frog without its brain has no Will and no consciousness; but its spinal cord when excited by afferent nerves, whose ends the vinegar stimulates, excites in turn efferent nerves which stimulate muscles, whose contraction produces a movement calculated to rub away the irritating object.
6. Now run a stout pin down the frog's neural canal so as to destroy its spinal cord. It will be found that no subsequent pinching of the creature or putting of vinegar on its skin causes any movement. Its muscles and nerve-trunks are intact, but the spinal reflex centre, which in the previous experiments was excited by afferent nerves, and then in turn stimulated efferent nerves, is destroyed. The heart continues to beat, on account of the automatic nerve-centres in it; but no voluntary and no reflex actions are exhibited by the animal.
7. The nerve-trunks and the muscles are, however, still active Turn the frog on its back and carefully expose (p. 331) the origins of the sciatic nerves. On pinching these, the muscles of the leg will be seen to contract. The irritation of the nerve by the pinch starts in it a nervous impulse which travels down the nerve-branches to the muscles.