We are able to judge to a certain extent of the order and kind of action of drugs upon the different parts of the nerve-centres by-watching their effect upon the movements of animals after their injection.

Functions of the Brain in the Frog

By removal of successive portions of the nervous system in the frog, Goltz has shown that the cerebral lobes have the function of voluntary movement, so that when they are removed, the animal lies quiet, unless acted upon by some external stimulus.

The optic lobes, which correspond to the corpora quadri-gemina of the higher animals, have the function of directing and co-ordinating movements, but not of originating them, so that a frog in which they are uninjured, but from which the cerebral lobes have been removed, will remain perfectly quiet, except on the application of an external stimulus, when it will leap like a normal frog.

As the optic lobes have the power of directing and co-ordinating movements, when they are destroyed the animal will jump, but will be unable to direct its movements.

The cerebellum has also the power of co-ordination, so that when it is removed the animal cannot jump at all, although one leg may answer by a kick or other motion to the application of a stimulus. But even when all those parts have been removed, the frog will still recover its ordinary position after it has been laid upon its back.

The co-ordination requisite for this power of retaining or recovering its ordinary position appears to be situated in the medulla oblongata, for when this is removed the frog will lie upon its back, and will not attempt to recover its ordinary position.

The legs will still respond by movements to irritation applied to the foot, but when the spinal cord is now destroyed these reflex movements also cease.

In frogs poisoned by opium, the movements are gradually abolished in the order just mentioned, and we therefore conclude that opium affects the nerve-centres in the order of their development, the highest being paralysed first, and the lowest last (p. 172). This order is usually not quite the same in higher animals, inasmuch as the last centre to be paralysed by opium or other anaesthetics is usually the medulla oblongata, and more especially that part of it which keeps up the respiratory movements. As we shall afterwards see, however, the respiratory centre is really a lower or more fundamental centre than either the brain or spinal cord.