620 



PHYSIOLOGY OF THE BRAIN-STEM 



some corner or wall and, at least so far as observed, never leaves its retreat except 

 when disturbed. 



After removal of the forebrain, 'tweenbrain and hindbrain the dogfish never 

 spontaneously makes any movements. Eoused artificially, its movements are 

 effective and perfectly regular, and, so long as it does not leave the normal plane 

 with reference to the direction of gravity, it keeps its balance properly. But let 

 it once get out of the normal position in the water, and its equilibrium is easily 



lost. It may even come to rest 

 lying on its back. When the ani- 

 mal is suddenly and forcibly placed 

 on its back it makes very evident 

 efforts to regain its normal posi- 

 tion, but does not always succeed. 

 Hence in the dogfish also the 

 so - called spontaneous movements 

 and the finer coordination of move- 

 ments appear to be bound up with 

 the 'tweenbrain and the midbrain. 

 The lower parts of the brain, how- 

 ever, are alone sufficient to carry 

 out fairly well coordinated move- 

 ments started by artificial means. 

 Schrader also removed the 

 cerebrum from frogs (Fig. 276) 

 without injuring the 'tweenbrain. 

 There was no noticeable effect: 

 the frogs moved about " sponta- 

 neously " from one place to an- 

 other, they swam like perfectly 

 normal animals; at the approach 

 of cold weather they burrowed into 

 the mud or under stones; or, pass- 

 ing the winter in the open, they adapted themselves to external conditions and 

 with the same results as did their normal companions. At the end of the 

 hibernating season, or in summer some months after the wound was perfectly 

 healed, the animals operated upon, just like the normal ones, caught all the flies 

 in the cage, and so on. 



But when the 'tweenbrain was injured along- with the cerebrum, the same 

 condition appeared as had formerly been described by Goltz as the consequence 

 of removing the cerebrum alone. There were no motor effects strictly speaking, 

 but the animals had lost all their spontaneity. When a frog in this condition 

 was not roused by some external stimulus, it would sit perfectly still, until it 

 dried up to a mummy ; it never tried to catch flies, no difference how many were 

 in the cage it starved to death in the midst of plenty, unless it was artificially 

 fed. Its movements were just like those of a normal frog except that they were 

 perfectly machinelike a given stimulus always giving the same response. Since 

 the optic nerves were left uninjured by the operation, the animal was influenced 

 by visual impressions, avoiding obstacles by going round them or jumping 

 over them. When it was lowered into and under the water very gradually, 

 by means of a mechanism driven by a screw, the stimulus of the change of 

 medium was not sufficient to cause the frog to move. It simply remained sus- 

 pended in the water at a depth determined only by the amount of air in the 

 lungs. 



Olfactory nerve 



Olfactory lobe 



Forebrain 

 (hemispheres) 



'Tweenbrain 



Midbrain 

 (optic lobes) 



Hindbrain 



(cerebellum) 

 Afterbrain 



(medulla oblongata) 



FIG. 276. The brain of a frog, after Steiner. 



